diff --git a/src/Assert.php b/src/Assert.php
index 74659b8..5451fe1 100644
--- a/src/Assert.php
+++ b/src/Assert.php
@@ -485,8 +485,8 @@ public static function isAOf($value, $class, $message = '')
 
         if (!\is_a($value, $class, \is_string($value))) {
             static::reportInvalidArgument(sprintf(
-                $message ?: 'Expected an instance of this class or to this class among his parents %2$s. Got: %s',
-                static::typeToString($value),
+                $message ?: 'Expected an instance of this class or to this class among its parents "%2$s". Got: %s',
+                static::valueToString($value),
                 $class
             ));
         }
@@ -511,8 +511,8 @@ public static function isNotA($value, $class, $message = '')
 
         if (\is_a($value, $class, \is_string($value))) {
             static::reportInvalidArgument(sprintf(
-                $message ?: 'Expected an instance of this class or to this class among his parents other than %2$s. Got: %s',
-                static::typeToString($value),
+                $message ?: 'Expected an instance of this class or to this class among its parents other than "%2$s". Got: %s',
+                static::valueToString($value),
                 $class
             ));
         }
@@ -539,9 +539,9 @@ public static function isAnyOf($value, array $classes, $message = '')
         }
 
         static::reportInvalidArgument(sprintf(
-            $message ?: 'Expected an any of instance of this class or to this class among his parents other than %2$s. Got: %s',
-            static::typeToString($value),
-            \implode(', ', \array_map(array('static', 'valueToString'), $classes))
+            $message ?: 'Expected an instance of any of this classes or any of those classes among their parents "%2$s". Got: %s',
+            static::valueToString($value),
+            \implode(', ', $classes)
         ));
     }
 
diff --git a/tests/AssertTest.php b/tests/AssertTest.php
index 4e2237e..c92b580 100644
--- a/tests/AssertTest.php
+++ b/tests/AssertTest.php
@@ -727,7 +727,7 @@ public function getStringConversions()
      */
     public function testConvertValuesToStrings($method, $args, $exceptionMessage)
     {
-        $this->expectException('\InvalidArgumentException', $exceptionMessage);
+        $this->expectException('\InvalidArgumentException');
         $this->expectExceptionMessage($exceptionMessage);
 
         call_user_func_array(array('Webmozart\Assert\Assert', $method), $args);
@@ -739,6 +739,45 @@ public function testAnUnknownMethodThrowsABadMethodCall()
 
         Assert::nonExistentMethod();
     }
+
+    public function getInvalidIsAOfCases(): iterable
+    {
+        yield array(
+            array('stdClass', 123),
+            'Expected class as a string. Got: integer',
+        );
+
+        yield array(
+            array('Iterator', 'ArrayIterator'),
+            'Expected an instance of this class or to this class among its parents "ArrayIterator". Got: "Iterator"',
+        );
+
+        yield array(
+            array(123, 'Iterator'),
+            'Expected an instance of this class or to this class among its parents "Iterator". Got: 123',
+        );
+
+        yield array(
+            array(array(), 'Iterator'),
+            'Expected an instance of this class or to this class among its parents "Iterator". Got: array',
+        );
+
+        yield array(
+            array(new \stdClass(), 'Iterator'),
+            'Expected an instance of this class or to this class among its parents "Iterator". Got: stdClass',
+        );
+    }
+
+    /**
+     * @dataProvider getInvalidIsAOfCases
+     */
+    public function testIsAOfExceptionMessages(array $args, string $exceptionMessage): void
+    {
+        $this->expectException('\InvalidArgumentException');
+        $this->expectExceptionMessage($exceptionMessage);
+
+        call_user_func_array(array('Webmozart\Assert\Assert', 'isAOf'), $args);
+    }
 }
 
 /**