diff --git a/tests/unit/Framework/MockObject/ReturnValueGeneratorTest.php b/tests/unit/Framework/MockObject/ReturnValueGeneratorTest.php
index 2545affceb5..6529d5854a0 100644
--- a/tests/unit/Framework/MockObject/ReturnValueGeneratorTest.php
+++ b/tests/unit/Framework/MockObject/ReturnValueGeneratorTest.php
@@ -22,6 +22,7 @@
 use PHPUnit\TestFixture\MockObject\AnInterfaceForIssue5593;
 use PHPUnit\TestFixture\MockObject\AnotherInterface;
 use PHPUnit\TestFixture\MockObject\AnotherInterfaceForIssue5593;
+use PHPUnit\TestFixture\MockObject\ExtendableClass;
 use PHPUnit\TestFixture\MockObject\YetAnotherInterface;
 use stdClass;
 
@@ -201,6 +202,22 @@ public function test_Generates_test_stub_for_first_intersection_of_interfaces_fo
         $this->assertInstanceOf(AnotherInterface::class, $value);
     }
 
+    public function test_Does_not_handle_union_of_extendable_class_and_interface(): void
+    {
+        $this->expectException(RuntimeException::class);
+        $this->expectExceptionMessage('Return value for OriginalClassName::methodName() cannot be generated because the declared return type is a union, please configure a return value for this method');
+
+        $this->generate(ExtendableClass::class . '|' . AnInterface::class);
+    }
+
+    public function test_Does_not_handle_intersection_of_extendable_class_and_interface(): void
+    {
+        $this->expectException(RuntimeException::class);
+        $this->expectExceptionMessage('Return value for OriginalClassName::methodName() cannot be generated because the declared return type is an intersection, please configure a return value for this method');
+
+        $this->generate(ExtendableClass::class . '&' . AnInterface::class);
+    }
+
     public function test_Generates_test_stub_for_unknown_type(): void
     {
         $this->assertInstanceOf(Stub::class, $this->generate('ThisDoesNotExist'));