diff --git a/CHANGELOG.md b/CHANGELOG.md
index 538c846..2449ee4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,6 +1,12 @@
 Changelog
 =========
 
+## 2.1.5
+
+### Fixed
+
+- Fixed regression of `instanceOf` messages
+
 ## 2.1.4
 
 ### Fixed
diff --git a/src/Assert.php b/src/Assert.php
index 65a45a9..24f9c1a 100644
--- a/src/Assert.php
+++ b/src/Assert.php
@@ -484,7 +484,6 @@ public static function isIterable(mixed $value, string $message = ''): iterable
      */
     public static function isInstanceOf(mixed $value, mixed $class, string $message = ''): object
     {
-        static::object($value, $message);
         static::string($class, 'Expected class as a string. Got: %s');
 
         if (!($value instanceof $class)) {
@@ -510,10 +509,9 @@ public static function isInstanceOf(mixed $value, mixed $class, string $message
      */
     public static function notInstanceOf(mixed $value, mixed $class, string $message = ''): object
     {
-        static::object($value, $message);
         static::string($class, 'Expected class as a string. Got: %s');
 
-        if ($value instanceof $class) {
+        if (!\is_object($value) || $value instanceof $class) {
             static::reportInvalidArgument(\sprintf(
                 $message ?: 'Expected an instance other than %2$s. Got: %s',
                 static::typeToString($value),
@@ -537,7 +535,6 @@ public static function notInstanceOf(mixed $value, mixed $class, string $message
      */
     public static function isInstanceOfAny(mixed $value, mixed $classes, string $message = ''): object
     {
-        static::object($value, $message);
         static::isIterable($classes);
 
         foreach ($classes as $class) {
diff --git a/tests/AssertTest.php b/tests/AssertTest.php
index 5b3d0a3..8fefdbc 100644
--- a/tests/AssertTest.php
+++ b/tests/AssertTest.php
@@ -188,7 +188,9 @@ public static function getTests(): array
             ['isInstanceOf', [null, 'stdClass'], false],
             ['notInstanceOf', [new stdClass(), 'stdClass'], false],
             ['notInstanceOf', [new Exception(), 'stdClass'], true],
+            ['notInstanceOf', [123, 'stdClass'], false],
             ['notInstanceOf', [[], 'stdClass'], false],
+            ['notInstanceOf', [null, 'stdClass'], false],
             ['isInstanceOfAny', [new ArrayIterator(), ['Iterator', 'ArrayAccess']], true],
             ['isInstanceOfAny', [new Exception(), ['Exception', 'Countable']], true],
             ['isInstanceOfAny', [new Exception(), ['ArrayAccess', 'Countable']], false],
@@ -803,6 +805,38 @@ public function testConvertValuesToStrings(string $method, array $args, string $
         call_user_func_array(['Webmozart\Assert\Assert', $method], $args);
     }
 
+    public static function getInvalidInstanceOfCases(): iterable
+    {
+        yield [
+            [null, 'stdClass'],
+            'Expected an instance of stdClass. Got: NULL',
+        ];
+
+        yield [
+            [123, 'stdClass'],
+            'Expected an instance of stdClass. Got: integer',
+        ];
+
+        yield [
+            [[], 'Iterator'],
+            'Expected an instance of Iterator. Got: array',
+        ];
+
+        yield [
+            [new stdClass(), 'Iterator'],
+            'Expected an instance of Iterator. Got: stdClass',
+        ];
+    }
+
+    #[DataProvider('getInvalidInstanceOfCases')]
+    public function testInstanceOfExceptionMessages(array $args, string $exceptionMessage): void
+    {
+        $this->expectException('\InvalidArgumentException');
+        $this->expectExceptionMessage($exceptionMessage);
+
+        call_user_func_array(['Webmozart\Assert\Assert', 'isInstanceOf'], $args);
+    }
+
     public static function getInvalidIsAOfCases(): iterable
     {
         yield [
@@ -955,7 +989,7 @@ public static function getMethodsThatUseOtherMethods(): array
             ],
             [
                 'method' => 'isInstanceOfAny',
-                'args' => [111, 'stdClass', 'Value must be an instance of stdClass. Got: %s'],
+                'args' => [111, ['stdClass'], 'Value must be an instance of stdClass. Got: %s'],
                 'exceptionMessage' => 'Value must be an instance of stdClass. Got: integer',
             ],
             [
@@ -965,7 +999,7 @@ public static function getMethodsThatUseOtherMethods(): array
             ],
             [
                 'method' => 'isAnyOf',
-                'args' => [111, 'stdClass', 'Value must be an instance of stdClass. Got: %s'],
+                'args' => [111, ['stdClass'], 'Value must be an instance of stdClass. Got: %s'],
                 'exceptionMessage' => 'Value must be an instance of stdClass. Got: integer',
             ],
         ];