diff --git a/spring-test/src/main/java/org/springframework/mock/web/MockHttpServletResponse.java b/spring-test/src/main/java/org/springframework/mock/web/MockHttpServletResponse.java
index f0daaf90a62..823945b5974 100644
--- a/spring-test/src/main/java/org/springframework/mock/web/MockHttpServletResponse.java
+++ b/spring-test/src/main/java/org/springframework/mock/web/MockHttpServletResponse.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -421,7 +421,7 @@ public class MockHttpServletResponse implements HttpServletResponse {
 
 	@Override
 	public boolean containsHeader(String name) {
-		return (this.headers.get(name) != null);
+		return this.headers.containsKey(name);
 	}
 
 	/**
@@ -594,12 +594,12 @@ public class MockHttpServletResponse implements HttpServletResponse {
 	}
 
 	@Override
-	public void setHeader(String name, String value) {
+	public void setHeader(String name, @Nullable String value) {
 		setHeaderValue(name, value);
 	}
 
 	@Override
-	public void addHeader(String name, String value) {
+	public void addHeader(String name, @Nullable String value) {
 		addHeaderValue(name, value);
 	}
 
@@ -613,7 +613,10 @@ public class MockHttpServletResponse implements HttpServletResponse {
 		addHeaderValue(name, value);
 	}
 
-	private void setHeaderValue(String name, Object value) {
+	private void setHeaderValue(String name, @Nullable Object value) {
+		if (value == null) {
+			return;
+		}
 		boolean replaceHeader = true;
 		if (setSpecialHeader(name, value, replaceHeader)) {
 			return;
@@ -621,7 +624,10 @@ public class MockHttpServletResponse implements HttpServletResponse {
 		doAddHeaderValue(name, value, replaceHeader);
 	}
 
-	private void addHeaderValue(String name, Object value) {
+	private void addHeaderValue(String name, @Nullable Object value) {
+		if (value == null) {
+			return;
+		}
 		boolean replaceHeader = false;
 		if (setSpecialHeader(name, value, replaceHeader)) {
 			return;
diff --git a/spring-test/src/test/java/org/springframework/mock/web/MockHttpServletResponseTests.java b/spring-test/src/test/java/org/springframework/mock/web/MockHttpServletResponseTests.java
index b105ea7953a..907a5cfac9f 100644
--- a/spring-test/src/test/java/org/springframework/mock/web/MockHttpServletResponseTests.java
+++ b/spring-test/src/test/java/org/springframework/mock/web/MockHttpServletResponseTests.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -26,6 +26,8 @@ import javax.servlet.http.Cookie;
 import javax.servlet.http.HttpServletResponse;
 
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
 
 import org.springframework.web.util.WebUtils;
 
@@ -57,6 +59,32 @@ class MockHttpServletResponseTests {
 	private MockHttpServletResponse response = new MockHttpServletResponse();
 
 
+	@ParameterizedTest  // gh-26488
+	@ValueSource(strings = {
+		CONTENT_TYPE,
+		CONTENT_LENGTH,
+		CONTENT_LANGUAGE,
+		SET_COOKIE,
+		"enigma"
+	})
+	void addHeaderWithNullValue(String headerName) {
+		response.addHeader(headerName, null);
+		assertThat(response.containsHeader(headerName)).isFalse();
+	}
+
+	@ParameterizedTest  // gh-26488
+	@ValueSource(strings = {
+		CONTENT_TYPE,
+		CONTENT_LENGTH,
+		CONTENT_LANGUAGE,
+		SET_COOKIE,
+		"enigma"
+	})
+	void setHeaderWithNullValue(String headerName) {
+		response.setHeader(headerName, null);
+		assertThat(response.containsHeader(headerName)).isFalse();
+	}
+
 	@Test
 	void setContentType() {
 		String contentType = "test/plain";
diff --git a/spring-web/src/testFixtures/java/org/springframework/web/testfixture/servlet/MockHttpServletResponse.java b/spring-web/src/testFixtures/java/org/springframework/web/testfixture/servlet/MockHttpServletResponse.java
index ed9bae93376..5af669a25ec 100644
--- a/spring-web/src/testFixtures/java/org/springframework/web/testfixture/servlet/MockHttpServletResponse.java
+++ b/spring-web/src/testFixtures/java/org/springframework/web/testfixture/servlet/MockHttpServletResponse.java
@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -421,7 +421,7 @@ public class MockHttpServletResponse implements HttpServletResponse {
 
 	@Override
 	public boolean containsHeader(String name) {
-		return (this.headers.get(name) != null);
+		return this.headers.containsKey(name);
 	}
 
 	/**
@@ -594,12 +594,12 @@ public class MockHttpServletResponse implements HttpServletResponse {
 	}
 
 	@Override
-	public void setHeader(String name, String value) {
+	public void setHeader(String name, @Nullable String value) {
 		setHeaderValue(name, value);
 	}
 
 	@Override
-	public void addHeader(String name, String value) {
+	public void addHeader(String name, @Nullable String value) {
 		addHeaderValue(name, value);
 	}
 
@@ -613,7 +613,10 @@ public class MockHttpServletResponse implements HttpServletResponse {
 		addHeaderValue(name, value);
 	}
 
-	private void setHeaderValue(String name, Object value) {
+	private void setHeaderValue(String name, @Nullable Object value) {
+		if (value == null) {
+			return;
+		}
 		boolean replaceHeader = true;
 		if (setSpecialHeader(name, value, replaceHeader)) {
 			return;
@@ -621,7 +624,10 @@ public class MockHttpServletResponse implements HttpServletResponse {
 		doAddHeaderValue(name, value, replaceHeader);
 	}
 
-	private void addHeaderValue(String name, Object value) {
+	private void addHeaderValue(String name, @Nullable Object value) {
+		if (value == null) {
+			return;
+		}
 		boolean replaceHeader = false;
 		if (setSpecialHeader(name, value, replaceHeader)) {
 			return;