diff --git a/spring-test/src/main/java/org/springframework/mock/http/server/reactive/MockServerHttpResponse.java b/spring-test/src/main/java/org/springframework/mock/http/server/reactive/MockServerHttpResponse.java
index f89cdfc07b0..3aa79c4a94a 100644
--- a/spring-test/src/main/java/org/springframework/mock/http/server/reactive/MockServerHttpResponse.java
+++ b/spring-test/src/main/java/org/springframework/mock/http/server/reactive/MockServerHttpResponse.java
@@ -50,9 +50,9 @@ public class MockServerHttpResponse implements ServerHttpResponse {
 
 	private final MultiValueMap<String, ResponseCookie> cookies = new LinkedMultiValueMap<>();
 
-	private Publisher<DataBuffer> body;
+	private Publisher<? extends DataBuffer> body;
 
-	private Publisher<? extends Publisher<DataBuffer>> bodyWithFlushes;
+	private Publisher<? extends Publisher<? extends DataBuffer>> bodyWithFlushes;
 
 	private DataBufferFactory bufferFactory = new DefaultDataBufferFactory();
 
@@ -77,22 +77,22 @@ public class MockServerHttpResponse implements ServerHttpResponse {
 		return this.cookies;
 	}
 
-	public Publisher<DataBuffer> getBody() {
+	public Publisher<? extends DataBuffer> getBody() {
 		return this.body;
 	}
 
-	public Publisher<? extends Publisher<DataBuffer>> getBodyWithFlush() {
+	public Publisher<? extends Publisher<? extends DataBuffer>> getBodyWithFlush() {
 		return this.bodyWithFlushes;
 	}
 
 	@Override
-	public Mono<Void> writeWith(Publisher<DataBuffer> body) {
+	public Mono<Void> writeWith(Publisher<? extends DataBuffer> body) {
 		this.body = body;
 		return Flux.from(this.body).then();
 	}
 
 	@Override
-	public Mono<Void> writeAndFlushWith(Publisher<? extends Publisher<DataBuffer>> body) {
+	public Mono<Void> writeAndFlushWith(Publisher<? extends Publisher<? extends DataBuffer>> body) {
 		this.bodyWithFlushes = body;
 		return Flux.from(this.bodyWithFlushes).then();
 	}
diff --git a/spring-web/src/main/java/org/springframework/http/ReactiveHttpOutputMessage.java b/spring-web/src/main/java/org/springframework/http/ReactiveHttpOutputMessage.java
index 043e884663b..e0f61a42310 100644
--- a/spring-web/src/main/java/org/springframework/http/ReactiveHttpOutputMessage.java
+++ b/spring-web/src/main/java/org/springframework/http/ReactiveHttpOutputMessage.java
@@ -55,7 +55,7 @@ public interface ReactiveHttpOutputMessage extends HttpMessage {
 	 * @param body the body content publisher
 	 * @return a {@link Mono} that indicates completion or error
 	 */
-	Mono<Void> writeWith(Publisher<DataBuffer> body);
+	Mono<Void> writeWith(Publisher<? extends DataBuffer> body);
 
 	/**
 	 * Use the given {@link Publisher} of {@code Publishers} to write the body of the
@@ -64,7 +64,7 @@ public interface ReactiveHttpOutputMessage extends HttpMessage {
 	 * @param body the body content publisher
 	 * @return a {@link Mono} that indicates completion or error
 	 */
-	Mono<Void> writeAndFlushWith(Publisher<? extends Publisher<DataBuffer>> body);
+	Mono<Void> writeAndFlushWith(Publisher<? extends Publisher<? extends DataBuffer>> body);
 
 	/**
 	 * Indicate that message handling is complete, allowing for any cleanup or
diff --git a/spring-web/src/main/java/org/springframework/http/client/reactive/ReactorClientHttpRequest.java b/spring-web/src/main/java/org/springframework/http/client/reactive/ReactorClientHttpRequest.java
index d5822617a6d..56a9cfb036a 100644
--- a/spring-web/src/main/java/org/springframework/http/client/reactive/ReactorClientHttpRequest.java
+++ b/spring-web/src/main/java/org/springframework/http/client/reactive/ReactorClientHttpRequest.java
@@ -74,20 +74,20 @@ public class ReactorClientHttpRequest extends AbstractClientHttpRequest {
 	}
 
 	@Override
-	public Mono<Void> writeWith(Publisher<DataBuffer> body) {
+	public Mono<Void> writeWith(Publisher<? extends DataBuffer> body) {
 		return applyBeforeCommit().then(this.httpRequest
 				.send(Flux.from(body).map(NettyDataBufferFactory::toByteBuf)));
 	}
 
 	@Override
-	public Mono<Void> writeAndFlushWith(Publisher<? extends Publisher<DataBuffer>> body) {
+	public Mono<Void> writeAndFlushWith(Publisher<? extends Publisher<? extends DataBuffer>> body) {
 		Publisher<Publisher<ByteBuf>> byteBufs = Flux.from(body).
 				map(ReactorClientHttpRequest::toByteBufs);
 		return applyBeforeCommit().then(this.httpRequest
 				.sendGroups(byteBufs));
 	}
 
-	private static Publisher<ByteBuf> toByteBufs(Publisher<DataBuffer> dataBuffers) {
+	private static Publisher<ByteBuf> toByteBufs(Publisher<? extends DataBuffer> dataBuffers) {
 		return Flux.from(dataBuffers).
 				map(NettyDataBufferFactory::toByteBuf);
 	}
diff --git a/spring-web/src/main/java/org/springframework/http/server/reactive/AbstractListenerServerHttpResponse.java b/spring-web/src/main/java/org/springframework/http/server/reactive/AbstractListenerServerHttpResponse.java
index e2558669e2f..35440503ed6 100644
--- a/spring-web/src/main/java/org/springframework/http/server/reactive/AbstractListenerServerHttpResponse.java
+++ b/spring-web/src/main/java/org/springframework/http/server/reactive/AbstractListenerServerHttpResponse.java
@@ -43,14 +43,14 @@ public abstract class AbstractListenerServerHttpResponse extends AbstractServerH
 
 
 	@Override
-	protected final Mono<Void> writeWithInternal(Publisher<DataBuffer> body) {
+	protected final Mono<Void> writeWithInternal(Publisher<? extends DataBuffer> body) {
 		return writeAndFlushWithInternal(Mono.just(body));
 	}
 
 	@Override
-	protected final Mono<Void> writeAndFlushWithInternal(Publisher<? extends Publisher<DataBuffer>> body) {
+	protected final Mono<Void> writeAndFlushWithInternal(Publisher<? extends Publisher<? extends DataBuffer>> body) {
 		if (this.writeCalled.compareAndSet(false, true)) {
-			Processor<Publisher<DataBuffer>, Void> bodyProcessor = createBodyFlushProcessor();
+			Processor<? super Publisher<? extends DataBuffer>, Void> bodyProcessor = createBodyFlushProcessor();
 			return Mono.from(subscriber -> {
 				body.subscribe(bodyProcessor);
 				bodyProcessor.subscribe(subscriber);
@@ -67,6 +67,6 @@ public abstract class AbstractListenerServerHttpResponse extends AbstractServerH
 	 * that will write the response body with flushes to the underlying output. Called from
 	 * {@link #writeAndFlushWithInternal(Publisher)}.
 	 */
-	protected abstract Processor<Publisher<DataBuffer>, Void> createBodyFlushProcessor();
+	protected abstract Processor<? super Publisher<? extends DataBuffer>, Void> createBodyFlushProcessor();
 
 }
diff --git a/spring-web/src/main/java/org/springframework/http/server/reactive/AbstractResponseBodyFlushProcessor.java b/spring-web/src/main/java/org/springframework/http/server/reactive/AbstractResponseBodyFlushProcessor.java
index 57510a9c094..56215d668a7 100644
--- a/spring-web/src/main/java/org/springframework/http/server/reactive/AbstractResponseBodyFlushProcessor.java
+++ b/spring-web/src/main/java/org/springframework/http/server/reactive/AbstractResponseBodyFlushProcessor.java
@@ -41,7 +41,7 @@ import org.springframework.core.io.buffer.DataBuffer;
  * @see UndertowHttpHandlerAdapter
  * @see ServerHttpResponse#writeAndFlushWith(Publisher)
  */
-abstract class AbstractResponseBodyFlushProcessor implements Processor<Publisher<DataBuffer>, Void> {
+abstract class AbstractResponseBodyFlushProcessor implements Processor<Publisher<? extends DataBuffer>, Void> {
 
 	protected final Log logger = LogFactory.getLog(getClass());
 
@@ -65,7 +65,7 @@ abstract class AbstractResponseBodyFlushProcessor implements Processor<Publisher
 	}
 
 	@Override
-	public final void onNext(Publisher<DataBuffer> publisher) {
+	public final void onNext(Publisher<? extends DataBuffer> publisher) {
 		if (logger.isTraceEnabled()) {
 			logger.trace(this.state + " onNext: " + publisher);
 		}
@@ -100,7 +100,7 @@ abstract class AbstractResponseBodyFlushProcessor implements Processor<Publisher
 	/**
 	 * Creates a new processor for subscribing to a body chunk.
 	 */
-	protected abstract Processor<DataBuffer, Void> createBodyProcessor();
+	protected abstract Processor<? super DataBuffer, Void> createBodyProcessor();
 
 	/**
 	 * Flushes the output.
@@ -144,9 +144,9 @@ abstract class AbstractResponseBodyFlushProcessor implements Processor<Publisher
 		REQUESTED {
 
 			@Override
-			public void onNext(AbstractResponseBodyFlushProcessor processor, Publisher<DataBuffer> chunk) {
+			public void onNext(AbstractResponseBodyFlushProcessor processor, Publisher<? extends DataBuffer> chunk) {
 				if (processor.changeState(this, RECEIVED)) {
-					Processor<DataBuffer, Void> chunkProcessor = processor.createBodyProcessor();
+					Processor<? super DataBuffer, Void> chunkProcessor = processor.createBodyProcessor();
 					chunk.subscribe(chunkProcessor);
 					chunkProcessor.subscribe(new WriteSubscriber(processor));
 				}
@@ -192,7 +192,7 @@ abstract class AbstractResponseBodyFlushProcessor implements Processor<Publisher
 
 			@Override
 			public void onNext(AbstractResponseBodyFlushProcessor processor,
-					Publisher<DataBuffer> publisher) {
+					Publisher<? extends DataBuffer> publisher) {
 				// ignore
 
 			}
@@ -212,7 +212,7 @@ abstract class AbstractResponseBodyFlushProcessor implements Processor<Publisher
 			subscription.cancel();
 		}
 
-		public void onNext(AbstractResponseBodyFlushProcessor processor, Publisher<DataBuffer> publisher) {
+		public void onNext(AbstractResponseBodyFlushProcessor processor, Publisher<? extends DataBuffer> publisher) {
 			throw new IllegalStateException(toString());
 		}
 
diff --git a/spring-web/src/main/java/org/springframework/http/server/reactive/AbstractServerHttpResponse.java b/spring-web/src/main/java/org/springframework/http/server/reactive/AbstractServerHttpResponse.java
index 519f5519341..a5163b22922 100644
--- a/spring-web/src/main/java/org/springframework/http/server/reactive/AbstractServerHttpResponse.java
+++ b/spring-web/src/main/java/org/springframework/http/server/reactive/AbstractServerHttpResponse.java
@@ -125,13 +125,13 @@ public abstract class AbstractServerHttpResponse implements ServerHttpResponse {
 	}
 
 	@Override
-	public final Mono<Void> writeWith(Publisher<DataBuffer> body) {
+	public final Mono<Void> writeWith(Publisher<? extends DataBuffer> body) {
 		return new ChannelSendOperator<>(body,
 				writePublisher -> doCommit(() -> writeWithInternal(writePublisher)));
 	}
 
 	@Override
-	public final Mono<Void> writeAndFlushWith(Publisher<? extends Publisher<DataBuffer>> body) {
+	public final Mono<Void> writeAndFlushWith(Publisher<? extends Publisher<? extends DataBuffer>> body) {
 		return new ChannelSendOperator<>(body,
 				writePublisher -> doCommit(() -> writeAndFlushWithInternal(writePublisher)));
 	}
@@ -186,14 +186,14 @@ public abstract class AbstractServerHttpResponse implements ServerHttpResponse {
 	 * Implement this method to write to the underlying the response.
 	 * @param body the publisher to write with
 	 */
-	protected abstract Mono<Void> writeWithInternal(Publisher<DataBuffer> body);
+	protected abstract Mono<Void> writeWithInternal(Publisher<? extends DataBuffer> body);
 
 	/**
 	 * Implement this method to write to the underlying the response, and flush after
 	 * each {@code Publisher<DataBuffer>}.
 	 * @param body the publisher to write and flush with
 	 */
-	protected abstract Mono<Void> writeAndFlushWithInternal(Publisher<? extends Publisher<DataBuffer>> body);
+	protected abstract Mono<Void> writeAndFlushWithInternal(Publisher<? extends Publisher<? extends DataBuffer>> body);
 
 	/**
 	 * Implement this method to write the status code to the underlying response.
diff --git a/spring-web/src/main/java/org/springframework/http/server/reactive/ReactorServerHttpResponse.java b/spring-web/src/main/java/org/springframework/http/server/reactive/ReactorServerHttpResponse.java
index c7711d32959..64c8a10f749 100644
--- a/spring-web/src/main/java/org/springframework/http/server/reactive/ReactorServerHttpResponse.java
+++ b/spring-web/src/main/java/org/springframework/http/server/reactive/ReactorServerHttpResponse.java
@@ -70,13 +70,13 @@ public class ReactorServerHttpResponse extends AbstractServerHttpResponse
 	}
 
 	@Override
-	protected Mono<Void> writeWithInternal(Publisher<DataBuffer> publisher) {
+	protected Mono<Void> writeWithInternal(Publisher<? extends DataBuffer> publisher) {
 		Publisher<ByteBuf> body = toByteBufs(publisher);
 		return this.response.send(body);
 	}
 
 	@Override
-	protected Mono<Void> writeAndFlushWithInternal(Publisher<? extends Publisher<DataBuffer>> publisher) {
+	protected Mono<Void> writeAndFlushWithInternal(Publisher<? extends Publisher<? extends DataBuffer>> publisher) {
 		Publisher<Publisher<ByteBuf>> body = Flux.from(publisher)
 				.map(ReactorServerHttpResponse::toByteBufs);
 		return this.response.sendGroups(body);
@@ -117,7 +117,7 @@ public class ReactorServerHttpResponse extends AbstractServerHttpResponse
 		return doCommit(() -> this.response.sendFile(file, position, count));
 	}
 
-	private static Publisher<ByteBuf> toByteBufs(Publisher<DataBuffer> dataBuffers) {
+	private static Publisher<ByteBuf> toByteBufs(Publisher<? extends DataBuffer> dataBuffers) {
 		return Flux.from(dataBuffers).map(NettyDataBufferFactory::toByteBuf);
 	}
 
diff --git a/spring-web/src/main/java/org/springframework/http/server/reactive/RxNettyServerHttpResponse.java b/spring-web/src/main/java/org/springframework/http/server/reactive/RxNettyServerHttpResponse.java
index 7b3df4df6be..4d2e37abbe5 100644
--- a/spring-web/src/main/java/org/springframework/http/server/reactive/RxNettyServerHttpResponse.java
+++ b/spring-web/src/main/java/org/springframework/http/server/reactive/RxNettyServerHttpResponse.java
@@ -71,7 +71,7 @@ public class RxNettyServerHttpResponse extends AbstractServerHttpResponse {
 	}
 
 	@Override
-	protected Mono<Void> writeWithInternal(Publisher<DataBuffer> body) {
+	protected Mono<Void> writeWithInternal(Publisher<? extends DataBuffer> body) {
 		Observable<ByteBuf> content = RxReactiveStreams.toObservable(body)
 				.map(NettyDataBufferFactory::toByteBuf);
 		return Flux.from(RxReactiveStreams.toPublisher(this.response.write(content)))
@@ -80,7 +80,7 @@ public class RxNettyServerHttpResponse extends AbstractServerHttpResponse {
 
 	@Override
 	protected Mono<Void> writeAndFlushWithInternal(
-			Publisher<? extends Publisher<DataBuffer>> body) {
+			Publisher<? extends Publisher<? extends DataBuffer>> body) {
 		Flux<ByteBuf> bodyWithFlushSignals = Flux.from(body).
 				flatMap(publisher -> Flux.from(publisher).
 						map(NettyDataBufferFactory::toByteBuf).
diff --git a/spring-web/src/main/java/org/springframework/http/server/reactive/ServletServerHttpResponse.java b/spring-web/src/main/java/org/springframework/http/server/reactive/ServletServerHttpResponse.java
index 7967d2dcb75..f6151aa3a7f 100644
--- a/spring-web/src/main/java/org/springframework/http/server/reactive/ServletServerHttpResponse.java
+++ b/spring-web/src/main/java/org/springframework/http/server/reactive/ServletServerHttpResponse.java
@@ -122,7 +122,7 @@ public class ServletServerHttpResponse extends AbstractListenerServerHttpRespons
 	}
 
 	@Override
-	protected Processor<Publisher<DataBuffer>, Void> createBodyFlushProcessor() {
+	protected Processor<? super Publisher<? extends DataBuffer>, Void> createBodyFlushProcessor() {
 		ResponseBodyFlushProcessor processor = new ResponseBodyFlushProcessor();
 		this.bodyFlushProcessor = processor;
 		return processor;
@@ -261,7 +261,7 @@ public class ServletServerHttpResponse extends AbstractListenerServerHttpRespons
 	private class ResponseBodyFlushProcessor extends AbstractResponseBodyFlushProcessor {
 
 		@Override
-		protected Processor<DataBuffer, Void> createBodyProcessor() {
+		protected Processor<? super DataBuffer, Void> createBodyProcessor() {
 			try {
 				bodyProcessor = new ResponseBodyProcessor(outputStream(), bufferSize);
 				return bodyProcessor;
diff --git a/spring-web/src/main/java/org/springframework/http/server/reactive/UndertowServerHttpResponse.java b/spring-web/src/main/java/org/springframework/http/server/reactive/UndertowServerHttpResponse.java
index b07b8cdad1e..9aad48d13ff 100644
--- a/spring-web/src/main/java/org/springframework/http/server/reactive/UndertowServerHttpResponse.java
+++ b/spring-web/src/main/java/org/springframework/http/server/reactive/UndertowServerHttpResponse.java
@@ -29,6 +29,7 @@ import io.undertow.server.handlers.Cookie;
 import io.undertow.server.handlers.CookieImpl;
 import io.undertow.util.HttpString;
 import org.reactivestreams.Processor;
+import org.reactivestreams.Publisher;
 import org.xnio.ChannelListener;
 import org.xnio.channels.StreamSinkChannel;
 import reactor.core.publisher.Mono;
@@ -123,7 +124,7 @@ public class UndertowServerHttpResponse extends AbstractListenerServerHttpRespon
 	}
 
 	@Override
-	protected AbstractResponseBodyFlushProcessor createBodyFlushProcessor() {
+	protected Processor<? super Publisher<? extends DataBuffer>, Void> createBodyFlushProcessor() {
 		return new ResponseBodyFlushProcessor();
 	}
 
@@ -209,7 +210,7 @@ public class UndertowServerHttpResponse extends AbstractListenerServerHttpRespon
 	private class ResponseBodyFlushProcessor extends AbstractResponseBodyFlushProcessor {
 
 		@Override
-		protected Processor<DataBuffer, Void> createBodyProcessor() {
+		protected Processor<? super DataBuffer, Void> createBodyProcessor() {
 			return UndertowServerHttpResponse.this.createBodyProcessor();
 		}
 
diff --git a/spring-web/src/test/java/org/springframework/http/codec/ServerSentEventHttpMessageWriterTests.java b/spring-web/src/test/java/org/springframework/http/codec/ServerSentEventHttpMessageWriterTests.java
index 33e566cf371..5789c629a8e 100644
--- a/spring-web/src/test/java/org/springframework/http/codec/ServerSentEventHttpMessageWriterTests.java
+++ b/spring-web/src/test/java/org/springframework/http/codec/ServerSentEventHttpMessageWriterTests.java
@@ -72,7 +72,7 @@ public class ServerSentEventHttpMessageWriterTests extends AbstractDataBufferAll
 		messageWriter.write(source, ResolvableType.forClass(ServerSentEvent.class),
 				new MediaType("text", "event-stream"), outputMessage, Collections.emptyMap());
 
-		Publisher<? extends Publisher<DataBuffer>> result = Flux.from(outputMessage.getBodyWithFlush());
+		Publisher<? extends Publisher<? extends DataBuffer>> result = Flux.from(outputMessage.getBodyWithFlush());
 		StepVerifier.create(result)
 				.consumeNextWith(sseConsumer("id:c42\n" + "event:foo\n" + "retry:123\n" +
 						":bla\n:bla bla\n:bla bla bla\n" + "data:bar\n"))
@@ -87,7 +87,7 @@ public class ServerSentEventHttpMessageWriterTests extends AbstractDataBufferAll
 		messageWriter.write(source, ResolvableType.forClass(String.class),
 				new MediaType("text", "event-stream"), outputMessage, Collections.emptyMap());
 
-		Publisher<? extends Publisher<DataBuffer>> result = outputMessage.getBodyWithFlush();
+		Publisher<? extends Publisher<? extends DataBuffer>> result = outputMessage.getBodyWithFlush();
 		StepVerifier.create(result)
 				.consumeNextWith(sseConsumer("data:foo\n"))
 				.consumeNextWith(sseConsumer("data:bar\n"))
@@ -102,7 +102,7 @@ public class ServerSentEventHttpMessageWriterTests extends AbstractDataBufferAll
 		messageWriter.write(source, ResolvableType.forClass(String.class),
 				new MediaType("text", "event-stream"), outputMessage, Collections.emptyMap());
 
-		Publisher<? extends Publisher<DataBuffer>> result = outputMessage.getBodyWithFlush();
+		Publisher<? extends Publisher<? extends DataBuffer>> result = outputMessage.getBodyWithFlush();
 		StepVerifier.create(result)
 				.consumeNextWith(sseConsumer("data:foo\ndata:bar\n"))
 				.consumeNextWith(sseConsumer("data:foo\ndata:baz\n"))
@@ -118,7 +118,7 @@ public class ServerSentEventHttpMessageWriterTests extends AbstractDataBufferAll
 		messageWriter.write(source, ResolvableType.forClass(Pojo.class),
 				new MediaType("text", "event-stream"), outputMessage, Collections.emptyMap());
 
-		Publisher<? extends Publisher<DataBuffer>> result = outputMessage.getBodyWithFlush();
+		Publisher<? extends Publisher<? extends DataBuffer>> result = outputMessage.getBodyWithFlush();
 		StepVerifier.create(result)
 				.consumeNextWith(sseConsumer("data:", "{\"foo\":\"foofoo\",\"bar\":\"barbar\"}", "\n"))
 				.consumeNextWith(sseConsumer("data:", "{\"foo\":\"foofoofoo\",\"bar\":\"barbarbar\"}", "\n"))
@@ -127,7 +127,7 @@ public class ServerSentEventHttpMessageWriterTests extends AbstractDataBufferAll
 	}
 
 
-	private Consumer<Publisher<DataBuffer>> sseConsumer(String... expected) {
+	private Consumer<Publisher<? extends DataBuffer>> sseConsumer(String... expected) {
 		return publisher -> {
 			StepVerifier.Step<DataBuffer> builder = StepVerifier.create(publisher);
 			for (String value : expected) {
diff --git a/spring-web/src/test/java/org/springframework/http/server/reactive/ServerHttpResponseTests.java b/spring-web/src/test/java/org/springframework/http/server/reactive/ServerHttpResponseTests.java
index 7c7e8fc2721..fd875411caa 100644
--- a/spring-web/src/test/java/org/springframework/http/server/reactive/ServerHttpResponseTests.java
+++ b/spring-web/src/test/java/org/springframework/http/server/reactive/ServerHttpResponseTests.java
@@ -27,6 +27,7 @@ import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 
 import org.springframework.core.io.buffer.DataBuffer;
+import org.springframework.core.io.buffer.DefaultDataBuffer;
 import org.springframework.core.io.buffer.DefaultDataBufferFactory;
 import org.springframework.http.ResponseCookie;
 
@@ -56,6 +57,20 @@ public class ServerHttpResponseTests {
 		assertEquals("c", new String(response.body.get(2).asByteBuffer().array(), StandardCharsets.UTF_8));
 	}
 
+	@Test  // SPR-14952
+	public void writeAndFlushWithFluxOfDefaultDataBuffer() throws Exception {
+		TestServerHttpResponse response = new TestServerHttpResponse();
+		Flux<Flux<DefaultDataBuffer>> flux = Flux.just(Flux.just(wrap("foo")));
+		response.writeAndFlushWith(flux).block();
+
+		assertTrue(response.statusCodeWritten);
+		assertTrue(response.headersWritten);
+		assertTrue(response.cookiesWritten);
+
+		assertEquals(1, response.body.size());
+		assertEquals("foo", new String(response.body.get(0).asByteBuffer().array(), StandardCharsets.UTF_8));
+	}
+
 	@Test
 	public void writeWithError() throws Exception {
 		TestServerHttpResponse response = new TestServerHttpResponse();
@@ -119,7 +134,7 @@ public class ServerHttpResponseTests {
 
 
 
-	private DataBuffer wrap(String a) {
+	private DefaultDataBuffer wrap(String a) {
 		return new DefaultDataBufferFactory().wrap(ByteBuffer.wrap(a.getBytes(StandardCharsets.UTF_8)));
 	}
 
@@ -157,7 +172,7 @@ public class ServerHttpResponseTests {
 		}
 
 		@Override
-		protected Mono<Void> writeWithInternal(Publisher<DataBuffer> body) {
+		protected Mono<Void> writeWithInternal(Publisher<? extends DataBuffer> body) {
 			return Flux.from(body).map(b -> {
 				this.body.add(b);
 				return b;
@@ -166,8 +181,13 @@ public class ServerHttpResponseTests {
 
 		@Override
 		protected Mono<Void> writeAndFlushWithInternal(
-				Publisher<? extends Publisher<DataBuffer>> body) {
-			return Mono.error(new UnsupportedOperationException());
+				Publisher<? extends Publisher<? extends DataBuffer>> bodyWithFlush) {
+			return Flux.from(bodyWithFlush).flatMap(body ->
+				Flux.from(body).map(b -> {
+					this.body.add(b);
+					return b;
+				})
+			).then();
 		}
 	}
 
diff --git a/spring-web/src/test/java/org/springframework/mock/http/server/reactive/test/MockServerHttpResponse.java b/spring-web/src/test/java/org/springframework/mock/http/server/reactive/test/MockServerHttpResponse.java
index c1f0f94b835..9105cd81ae4 100644
--- a/spring-web/src/test/java/org/springframework/mock/http/server/reactive/test/MockServerHttpResponse.java
+++ b/spring-web/src/test/java/org/springframework/mock/http/server/reactive/test/MockServerHttpResponse.java
@@ -49,9 +49,9 @@ public class MockServerHttpResponse implements ServerHttpResponse {
 
 	private final MultiValueMap<String, ResponseCookie> cookies = new LinkedMultiValueMap<>();
 
-	private Publisher<DataBuffer> body;
+	private Publisher<? extends DataBuffer> body;
 
-	private Publisher<? extends Publisher<DataBuffer>> bodyWithFlushes;
+	private Publisher<? extends Publisher<? extends DataBuffer>> bodyWithFlushes;
 
 	private DataBufferFactory bufferFactory = new DefaultDataBufferFactory();
 
@@ -77,22 +77,22 @@ public class MockServerHttpResponse implements ServerHttpResponse {
 		return this.cookies;
 	}
 
-	public Publisher<DataBuffer> getBody() {
+	public Publisher<? extends DataBuffer> getBody() {
 		return this.body;
 	}
 
-	public Publisher<? extends Publisher<DataBuffer>> getBodyWithFlush() {
+	public Publisher<? extends Publisher<? extends DataBuffer>> getBodyWithFlush() {
 		return this.bodyWithFlushes;
 	}
 
 	@Override
-	public Mono<Void> writeWith(Publisher<DataBuffer> body) {
+	public Mono<Void> writeWith(Publisher<? extends DataBuffer> body) {
 		this.body = body;
 		return Flux.from(this.body).then();
 	}
 
 	@Override
-	public Mono<Void> writeAndFlushWith(Publisher<? extends Publisher<DataBuffer>> body) {
+	public Mono<Void> writeAndFlushWith(Publisher<? extends Publisher<? extends DataBuffer>> body) {
 		this.bodyWithFlushes = body;
 		return Flux.from(this.bodyWithFlushes).then();
 	}
diff --git a/spring-web/src/test/java/org/springframework/web/client/reactive/test/MockClientHttpRequest.java b/spring-web/src/test/java/org/springframework/web/client/reactive/test/MockClientHttpRequest.java
index bf51e6c4c79..bffbf351b2a 100644
--- a/spring-web/src/test/java/org/springframework/web/client/reactive/test/MockClientHttpRequest.java
+++ b/spring-web/src/test/java/org/springframework/web/client/reactive/test/MockClientHttpRequest.java
@@ -41,9 +41,9 @@ public class MockClientHttpRequest extends AbstractClientHttpRequest {
 
 	private final DataBufferFactory bufferFactory = new DefaultDataBufferFactory();
 
-	private Publisher<DataBuffer> body;
+	private Publisher<? extends DataBuffer> body;
 
-	private Publisher<? extends Publisher<DataBuffer>> bodyWithFlushes;
+	private Publisher<? extends Publisher<? extends DataBuffer>> bodyWithFlushes;
 
 
 	public MockClientHttpRequest() {
@@ -90,22 +90,22 @@ public class MockClientHttpRequest extends AbstractClientHttpRequest {
 	}
 
 	@Override
-	public Mono<Void> writeWith(Publisher<DataBuffer> body) {
+	public Mono<Void> writeWith(Publisher<? extends DataBuffer> body) {
 		this.body = body;
 		return applyBeforeCommit().then(Flux.from(this.body).then());
 	}
 
 	@Override
-	public Mono<Void> writeAndFlushWith(Publisher<? extends Publisher<DataBuffer>> body) {
+	public Mono<Void> writeAndFlushWith(Publisher<? extends Publisher<? extends DataBuffer>> body) {
 		this.bodyWithFlushes = body;
 		return applyBeforeCommit().then(Flux.from(this.bodyWithFlushes).then());
 	}
 
-	public Publisher<DataBuffer> getBody() {
+	public Publisher<? extends DataBuffer> getBody() {
 		return body;
 	}
 
-	public Publisher<? extends Publisher<DataBuffer>> getBodyWithFlush() {
+	public Publisher<? extends Publisher<? extends DataBuffer>> getBodyWithFlush() {
 		return bodyWithFlushes;
 	}