diff --git a/spring-web/src/main/java/org/springframework/http/server/reactive/UndertowHeadersAdapter.java b/spring-web/src/main/java/org/springframework/http/server/reactive/UndertowHeadersAdapter.java
index bbe1f3df6e0..20dee910580 100644
--- a/spring-web/src/main/java/org/springframework/http/server/reactive/UndertowHeadersAdapter.java
+++ b/spring-web/src/main/java/org/springframework/http/server/reactive/UndertowHeadersAdapter.java
@@ -17,6 +17,7 @@
 package org.springframework.http.server.reactive;
 
 import java.util.AbstractSet;
+import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Iterator;
 import java.util.LinkedHashMap;
@@ -36,6 +37,7 @@ import org.springframework.util.MultiValueMap;
  * {@code MultiValueMap} implementation for wrapping Undertow HTTP headers.
  *
  * @author Brian Clozel
+ * @author Sam Brannen
  * @since 5.1.1
  */
 class UndertowHeadersAdapter implements MultiValueMap<String, String> {
@@ -131,7 +133,10 @@ class UndertowHeadersAdapter implements MultiValueMap<String, String> {
 	@Nullable
 	public List<String> remove(Object key) {
 		if (key instanceof String) {
-			this.headers.remove((String) key);
+			Collection<String> removed = this.headers.remove((String) key);
+			if (removed != null) {
+				return new ArrayList<>(removed);
+			}
 		}
 		return null;
 	}