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/ClientHttpConnectorBuilder.java b/module/spring-boot-http-client/src/main/java/org/springframework/boot/http/client/reactive/ClientHttpConnectorBuilder.java
index c892754be59..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));
 	}
 
@@ -139,7 +140,7 @@ public interface ClientHttpConnectorBuilder<T extends ClientHttpConnector> {
 	 */
 	@SuppressWarnings("unchecked")
 	static <T extends ClientHttpConnector> ClientHttpConnectorBuilder<T> of(Class<T> clientHttpConnectorType) {
-		Assert.notNull(clientHttpConnectorType, "'clientHttpConnectorType' must not be null");
+		Assert.notNull(clientHttpConnectorType, "'requestFactoryType' must not be null");
 		Assert.isTrue(clientHttpConnectorType != ClientHttpConnector.class,
 				"'clientHttpConnectorType' must be an implementation of ClientHttpConnector");
 		if (clientHttpConnectorType == ReactorClientHttpConnector.class) {
diff --git a/module/spring-boot-http-client/src/main/java/org/springframework/boot/http/client/reactive/HttpComponentsClientHttpConnectorBuilder.java b/module/spring-boot-http-client/src/main/java/org/springframework/boot/http/client/reactive/HttpComponentsClientHttpConnectorBuilder.java
index 6eff3a3c598..d47f8c424ed 100644
--- a/module/spring-boot-http-client/src/main/java/org/springframework/boot/http/client/reactive/HttpComponentsClientHttpConnectorBuilder.java
+++ b/module/spring-boot-http-client/src/main/java/org/springframework/boot/http/client/reactive/HttpComponentsClientHttpConnectorBuilder.java
@@ -66,7 +66,7 @@ public final class HttpComponentsClientHttpConnectorBuilder
 	 */
 	public HttpComponentsClientHttpConnectorBuilder withHttpClientCustomizer(
 			Consumer<HttpAsyncClientBuilder> httpClientCustomizer) {
-		Assert.notNull(httpClientCustomizer, "'httpClientCustomizer' must not be null");
+		Assert.notNull(httpClientCustomizer, "'customizer' must not be null");
 		return new HttpComponentsClientHttpConnectorBuilder(getCustomizers(),
 				this.httpClientBuilder.withCustomizer(httpClientCustomizer));
 	}
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 2b13595e11e..fab586a415a 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
@@ -54,6 +54,7 @@ import org.springframework.util.StreamUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
+import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
 
 /**
  * Base class for {@link ClientHttpRequestFactoryBuilder} tests.
@@ -76,6 +77,13 @@ 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/SimpleClientHttpRequestFactoryBuilderTests.java b/module/spring-boot-http-client/src/test/java/org/springframework/boot/http/client/SimpleClientHttpRequestFactoryBuilderTests.java
index 191d5f5b237..1b134981adf 100644
--- a/module/spring-boot-http-client/src/test/java/org/springframework/boot/http/client/SimpleClientHttpRequestFactoryBuilderTests.java
+++ b/module/spring-boot-http-client/src/test/java/org/springframework/boot/http/client/SimpleClientHttpRequestFactoryBuilderTests.java
@@ -16,7 +16,6 @@
 
 package org.springframework.boot.http.client;
 
-import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.ValueSource;
 
@@ -26,7 +25,6 @@ import org.springframework.http.client.SimpleClientHttpRequestFactory;
 import org.springframework.test.util.ReflectionTestUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
-import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
 import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
 
 /**
@@ -83,14 +81,6 @@ class SimpleClientHttpRequestFactoryBuilderTests
 		super.redirectDontFollow(httpMethod);
 	}
 
-	@Test
-	@SuppressWarnings("NullAway") // Test null check
-	void withCustomizerWhenCustomizerIsNullThrowsException() {
-		assertThatIllegalArgumentException()
-				.isThrownBy(() -> ClientHttpRequestFactoryBuilder.simple().withCustomizer(null))
-				.withMessage("'customizer' must not be null");
-	}
-
 	@Override
 	protected HttpStatus getExpectedRedirect(HttpMethod httpMethod) {
 		return (httpMethod != HttpMethod.GET) ? HttpStatus.FOUND : HttpStatus.OK;
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 0ebdfb443e1..8d8998f84e8 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
@@ -56,6 +56,7 @@ import org.springframework.web.reactive.function.client.WebClientRequestExceptio
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatExceptionOfType;
+import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
 
 /**
  * Base class for {@link ClientHttpConnectorBuilder} tests.
@@ -77,6 +78,13 @@ 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();
diff --git a/module/spring-boot-http-client/src/test/java/org/springframework/boot/http/client/reactive/HttpComponentsClientHttpConnectorBuilderTests.java b/module/spring-boot-http-client/src/test/java/org/springframework/boot/http/client/reactive/HttpComponentsClientHttpConnectorBuilderTests.java
index e6b250fe56b..0b5a7857478 100644
--- a/module/spring-boot-http-client/src/test/java/org/springframework/boot/http/client/reactive/HttpComponentsClientHttpConnectorBuilderTests.java
+++ b/module/spring-boot-http-client/src/test/java/org/springframework/boot/http/client/reactive/HttpComponentsClientHttpConnectorBuilderTests.java
@@ -39,7 +39,6 @@ import org.springframework.http.client.reactive.HttpComponentsClientHttpConnecto
 import org.springframework.test.util.ReflectionTestUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
-import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
 
 /**
  * Tests for {@link HttpComponentsClientHttpConnectorBuilder} and
@@ -81,14 +80,6 @@ class HttpComponentsClientHttpConnectorBuilderTests
 		defaultRequestConfigCustomizer1.assertCalled();
 	}
 
-	@Test
-	@SuppressWarnings("NullAway") // Test null check
-	void withHttpClientCustomizerWhenCustomizerIsNullThrowsException() {
-		assertThatIllegalArgumentException()
-			.isThrownBy(() -> ClientHttpConnectorBuilder.httpComponents().withHttpClientCustomizer(null))
-			.withMessage("'httpClientCustomizer' must not be null");
-	}
-
 	@Test
 	@WithPackageResources("test.jks")
 	void withTlsSocketStrategyFactory() {
diff --git a/module/spring-boot-http-client/src/test/java/org/springframework/boot/http/client/reactive/JdkClientHttpConnectorBuilderTests.java b/module/spring-boot-http-client/src/test/java/org/springframework/boot/http/client/reactive/JdkClientHttpConnectorBuilderTests.java
index 23c802a6d71..4af086153a5 100644
--- a/module/spring-boot-http-client/src/test/java/org/springframework/boot/http/client/reactive/JdkClientHttpConnectorBuilderTests.java
+++ b/module/spring-boot-http-client/src/test/java/org/springframework/boot/http/client/reactive/JdkClientHttpConnectorBuilderTests.java
@@ -28,7 +28,6 @@ import org.springframework.http.client.reactive.JdkClientHttpConnector;
 import org.springframework.test.util.ReflectionTestUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
-import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
 
 /**
  * Tests for {@link JdkClientHttpConnectorBuilder} and {@link JdkHttpClientBuilder}.
@@ -53,13 +52,6 @@ class JdkClientHttpConnectorBuilderTests extends AbstractClientHttpConnectorBuil
 		httpClientCustomizer2.assertCalled();
 	}
 
-	@Test
-	@SuppressWarnings("NullAway") // Test null check
-	void withCustomizerWhenCustomizerIsNullThrowsException() {
-		assertThatIllegalArgumentException().isThrownBy(() -> ClientHttpConnectorBuilder.jdk().withCustomizer(null))
-			.withMessage("'customizer' must not be null");
-	}
-
 	@Test
 	void withExecutor() {
 		Executor executor = new SimpleAsyncTaskExecutor();