diff --git a/module/spring-boot-http-client/src/main/java/org/springframework/boot/http/client/AbstractClientHttpRequestFactoryBuilder.java b/module/spring-boot-http-client/src/main/java/org/springframework/boot/http/client/AbstractClientHttpRequestFactoryBuilder.java
index 07c1c4c8fc1..fb34423aed2 100644
--- a/module/spring-boot-http-client/src/main/java/org/springframework/boot/http/client/AbstractClientHttpRequestFactoryBuilder.java
+++ b/module/spring-boot-http-client/src/main/java/org/springframework/boot/http/client/AbstractClientHttpRequestFactoryBuilder.java
@@ -48,7 +48,7 @@ abstract class AbstractClientHttpRequestFactoryBuilder<T extends ClientHttpReque
 	}
 
 	protected final List<Consumer<T>> mergedCustomizers(Consumer<T> customizer) {
-		Assert.notNull(this.customizers, "'customizer' must not be null");
+		Assert.notNull(customizer, "'customizer' must not be null");
 		return merge(this.customizers, List.of(customizer));
 	}
 
diff --git a/module/spring-boot-http-client/src/main/java/org/springframework/boot/http/client/ClientHttpRequestFactoryBuilder.java b/module/spring-boot-http-client/src/main/java/org/springframework/boot/http/client/ClientHttpRequestFactoryBuilder.java
index d0cd096fdd0..fdef32cf11b 100644
--- a/module/spring-boot-http-client/src/main/java/org/springframework/boot/http/client/ClientHttpRequestFactoryBuilder.java
+++ b/module/spring-boot-http-client/src/main/java/org/springframework/boot/http/client/ClientHttpRequestFactoryBuilder.java
@@ -70,6 +70,7 @@ public interface ClientHttpRequestFactoryBuilder<T extends ClientHttpRequestFact
 	 * @return a new {@link ClientHttpRequestFactoryBuilder} instance
 	 */
 	default ClientHttpRequestFactoryBuilder<T> withCustomizer(Consumer<T> customizer) {
+		Assert.notNull(customizer, "'customizer' must not be null");
 		return withCustomizers(List.of(customizer));
 	}
 
diff --git a/module/spring-boot-http-client/src/main/java/org/springframework/boot/http/client/reactive/AbstractClientHttpConnectorBuilder.java b/module/spring-boot-http-client/src/main/java/org/springframework/boot/http/client/reactive/AbstractClientHttpConnectorBuilder.java
index a9e8d0f9e04..573becc5b2d 100644
--- a/module/spring-boot-http-client/src/main/java/org/springframework/boot/http/client/reactive/AbstractClientHttpConnectorBuilder.java
+++ b/module/spring-boot-http-client/src/main/java/org/springframework/boot/http/client/reactive/AbstractClientHttpConnectorBuilder.java
@@ -49,7 +49,7 @@ abstract class AbstractClientHttpConnectorBuilder<T extends ClientHttpConnector>
 	}
 
 	protected final List<Consumer<T>> mergedCustomizers(Consumer<T> customizer) {
-		Assert.notNull(this.customizers, "'customizer' must not be null");
+		Assert.notNull(customizer, "'customizer' must not be null");
 		return merge(this.customizers, List.of(customizer));
 	}
 
diff --git a/module/spring-boot-http-client/src/main/java/org/springframework/boot/http/client/reactive/ClientHttpConnectorBuilder.java b/module/spring-boot-http-client/src/main/java/org/springframework/boot/http/client/reactive/ClientHttpConnectorBuilder.java
index 5ea306737f5..89cbfcbb27c 100644
--- a/module/spring-boot-http-client/src/main/java/org/springframework/boot/http/client/reactive/ClientHttpConnectorBuilder.java
+++ b/module/spring-boot-http-client/src/main/java/org/springframework/boot/http/client/reactive/ClientHttpConnectorBuilder.java
@@ -68,6 +68,7 @@ public interface ClientHttpConnectorBuilder<T extends ClientHttpConnector> {
 	 * @return a new {@link ClientHttpConnectorBuilder} instance
 	 */
 	default ClientHttpConnectorBuilder<T> withCustomizer(Consumer<T> customizer) {
+		Assert.notNull(customizer, "'customizer' must not be null");
 		return withCustomizers(List.of(customizer));
 	}
 
diff --git a/module/spring-boot-http-client/src/test/java/org/springframework/boot/http/client/AbstractClientHttpRequestFactoryBuilderTests.java b/module/spring-boot-http-client/src/test/java/org/springframework/boot/http/client/AbstractClientHttpRequestFactoryBuilderTests.java
index e2930eaec5a..afca60dda0c 100644
--- a/module/spring-boot-http-client/src/test/java/org/springframework/boot/http/client/AbstractClientHttpRequestFactoryBuilderTests.java
+++ b/module/spring-boot-http-client/src/test/java/org/springframework/boot/http/client/AbstractClientHttpRequestFactoryBuilderTests.java
@@ -55,6 +55,7 @@ import org.springframework.util.StreamUtils;
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatException;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
+import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
 
 /**
  * Base class for {@link ClientHttpRequestFactoryBuilder} tests.
@@ -77,6 +78,13 @@ abstract class AbstractClientHttpRequestFactoryBuilderTests<T extends ClientHttp
 		this.builder = builder;
 	}
 
+	@Test
+	@SuppressWarnings("NullAway") // Test null check
+	void withCustomizerWhenCustomizerIsNullThrowsException() {
+		assertThatIllegalArgumentException().isThrownBy(() -> this.builder.withCustomizer(null))
+			.withMessage("'customizer' must not be null");
+	}
+
 	@Test
 	void buildReturnsRequestFactoryOfExpectedType() {
 		T requestFactory = this.builder.build();
diff --git a/module/spring-boot-http-client/src/test/java/org/springframework/boot/http/client/reactive/AbstractClientHttpConnectorBuilderTests.java b/module/spring-boot-http-client/src/test/java/org/springframework/boot/http/client/reactive/AbstractClientHttpConnectorBuilderTests.java
index f0c4097a289..ca266f40c9f 100644
--- a/module/spring-boot-http-client/src/test/java/org/springframework/boot/http/client/reactive/AbstractClientHttpConnectorBuilderTests.java
+++ b/module/spring-boot-http-client/src/test/java/org/springframework/boot/http/client/reactive/AbstractClientHttpConnectorBuilderTests.java
@@ -59,6 +59,7 @@ import org.springframework.web.reactive.function.client.WebClientRequestExceptio
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatException;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
+import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
 
 /**
  * Base class for {@link ClientHttpConnectorBuilder} tests.
@@ -80,6 +81,13 @@ abstract class AbstractClientHttpConnectorBuilderTests<T extends ClientHttpConne
 		this.builder = builder;
 	}
 
+	@Test
+	@SuppressWarnings("NullAway") // Test null check
+	void withCustomizerWhenCustomizerIsNullThrowsException() {
+		assertThatIllegalArgumentException().isThrownBy(() -> this.builder.withCustomizer(null))
+			.withMessage("'customizer' must not be null");
+	}
+
 	@Test
 	void buildReturnsConnectorOfExpectedType() {
 		T connector = this.builder.build();