diff --git a/spring-core/src/main/java/org/springframework/aot/generate/GeneratedFiles.java b/spring-core/src/main/java/org/springframework/aot/generate/GeneratedFiles.java
index bd6ec60ae27c..1c326e4fd96d 100644
--- a/spring-core/src/main/java/org/springframework/aot/generate/GeneratedFiles.java
+++ b/spring-core/src/main/java/org/springframework/aot/generate/GeneratedFiles.java
@@ -20,6 +20,7 @@
 import org.springframework.javapoet.JavaFile;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
+import org.springframework.util.StringUtils;
 import org.springframework.util.function.ThrowingConsumer;
 
 /**
@@ -43,6 +44,7 @@ public interface GeneratedFiles {
 	 * @param javaFile the java file to add
 	 */
 	default void addSourceFile(JavaFile javaFile) {
+		validatePackage(javaFile.packageName, javaFile.typeSpec.name);
 		String className = javaFile.packageName + "." + javaFile.typeSpec.name;
 		addSourceFile(className, javaFile::writeTo);
 	}
@@ -161,11 +163,20 @@ default void addFile(Kind kind, String path, ThrowingConsumer<Appendable> conten
 
 	private static String getClassNamePath(String className) {
 		Assert.hasLength(className, "'className' must not be empty");
+		validatePackage(ClassUtils.getPackageName(className), className);
 		Assert.isTrue(isJavaIdentifier(className),
 				"'className' must be a valid identifier, got '" + className + "'");
 		return ClassUtils.convertClassNameToResourcePath(className) + ".java";
 	}
 
+	private static void validatePackage(String packageName, String className) {
+		if (!StringUtils.hasLength(packageName)) {
+			throw new IllegalArgumentException("Could not add '" + className + "', "
+					+ "processing classes in the default package is not supported. "
+					+ "Did you forget to add a package statement?");
+		}
+	}
+
 	private static boolean isJavaIdentifier(String className) {
 		char[] chars = className.toCharArray();
 		for (int i = 0; i < chars.length; i++) {
diff --git a/spring-core/src/test/java/org/springframework/aot/generate/GeneratedFilesTests.java b/spring-core/src/test/java/org/springframework/aot/generate/GeneratedFilesTests.java
index 0eeb510490e9..3372fe0c4dae 100644
--- a/spring-core/src/test/java/org/springframework/aot/generate/GeneratedFilesTests.java
+++ b/spring-core/src/test/java/org/springframework/aot/generate/GeneratedFilesTests.java
@@ -60,6 +60,15 @@ void addSourceFileWithJavaFileAddsFile() throws Exception {
 				.contains("Hello, World!");
 	}
 
+	@Test
+	void addSourceFileWithJavaFileInTheDefaultPackageThrowsException() {
+		TypeSpec helloWorld = TypeSpec.classBuilder("HelloWorld").build();
+		JavaFile javaFile = JavaFile.builder("", helloWorld).build();
+		assertThatIllegalArgumentException().isThrownBy(() -> this.generatedFiles.addSourceFile(javaFile))
+				.withMessage("Could not add 'HelloWorld', processing classes in the "
+						+ "default package is not supported. Did you forget to add a package statement?");
+	}
+
 	@Test
 	void addSourceFileWithCharSequenceAddsFile() throws Exception {
 		this.generatedFiles.addSourceFile("com.example.HelloWorld", "{}");
@@ -73,6 +82,14 @@ void addSourceFileWithCharSequenceWhenClassNameIsEmptyThrowsException() {
 				.withMessage("'className' must not be empty");
 	}
 
+	@Test
+	void addSourceFileWithCharSequenceWhenClassNameIsInTheDefaultPackageThrowsException() {
+		assertThatIllegalArgumentException()
+				.isThrownBy(() -> this.generatedFiles.addSourceFile("HelloWorld", "{}"))
+				.withMessage("Could not add 'HelloWorld', processing classes in the "
+						+ "default package is not supported. Did you forget to add a package statement?");
+	}
+
 	@Test
 	void addSourceFileWithCharSequenceWhenClassNameIsInvalidThrowsException() {
 		assertThatIllegalArgumentException()