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/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..c892754be59 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
@@ -139,7 +139,7 @@ public interface ClientHttpConnectorBuilder<T extends ClientHttpConnector> {
 	 */
 	@SuppressWarnings("unchecked")
 	static <T extends ClientHttpConnector> ClientHttpConnectorBuilder<T> of(Class<T> clientHttpConnectorType) {
-		Assert.notNull(clientHttpConnectorType, "'requestFactoryType' must not be null");
+		Assert.notNull(clientHttpConnectorType, "'clientHttpConnectorType' 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 d47f8c424ed..6eff3a3c598 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, "'customizer' must not be null");
+		Assert.notNull(httpClientCustomizer, "'httpClientCustomizer' 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/SimpleClientHttpRequestFactoryBuilderTests.java b/module/spring-boot-http-client/src/test/java/org/springframework/boot/http/client/SimpleClientHttpRequestFactoryBuilderTests.java
index 1b134981adf..191d5f5b237 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
@@ -16,6 +16,7 @@
 
 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;
 
@@ -25,6 +26,7 @@ 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;
 
 /**
@@ -81,6 +83,14 @@ 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/HttpComponentsClientHttpConnectorBuilderTests.java b/module/spring-boot-http-client/src/test/java/org/springframework/boot/http/client/reactive/HttpComponentsClientHttpConnectorBuilderTests.java
index 0b5a7857478..e6b250fe56b 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
@@ -39,6 +39,7 @@ 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
@@ -80,6 +81,14 @@ 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 4af086153a5..23c802a6d71 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
@@ -28,6 +28,7 @@ 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}.
@@ -52,6 +53,13 @@ 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();