diff --git a/Tests/Functional/Tools/Mapper/MapperTest.php b/Tests/Functional/Tools/Mapper/MapperTest.php
index 4b542639..5f5cefc3 100644
--- a/Tests/Functional/Tools/Mapper/MapperTest.php
+++ b/Tests/Functional/Tools/Mapper/MapperTest.php
@@ -37,7 +37,7 @@ public function dataProviderForCorrectMappings()
                         'x' => "obj.get('x') + 1",
                         'y' => "obj.get('x') + 2",
                         'z' => "obj.get('x') + 3",
-                        'array' => "obj.get('a')"
+                        'array' => "obj.get('a')",
                     ],
                 ],
                 'mapping_name' => 'x_to_xyz',
@@ -47,7 +47,7 @@ public function dataProviderForCorrectMappings()
                     'x' => 11,
                     'y' => 12,
                     'z' => 13,
-                    'array' => []
+                    'array' => [],
                 ],
             ],
             'Test null values' => [
@@ -55,7 +55,7 @@ public function dataProviderForCorrectMappings()
                     'mapping_name' => [
                         'x' => "obj.get('x') + 1",
                         'y' => "obj.get('y')",
-                        'z' => "obj.get('y').get('z')"
+                        'z' => "obj.get('y').get('z')",
                     ],
                 ],
                 'mapping_name' => 'mapping_name',
@@ -123,6 +123,23 @@ public function dataProviderForCorrectMappings()
                     'date_3' => '2015-01-01T20:00:00.000',
                 ],
             ],
+            'Test keyExists' => [
+                'mappings' => [
+                    'mapping_name' => [
+                        'test_key_1' => "mapper.keyExists(obj.get('test_array'), 'A')",
+                        'test_key_2' => "mapper.keyExists(obj.get('test_array'), 'C')",
+                    ],
+                ],
+                'mapping_name' => 'mapping_name',
+                'mapped_values' => new SerializableArray([
+                    'test_array' => ['A' => 0, 'B' => 0],
+                ]),
+                'context' => [],
+                'expected_value' => [
+                    'test_key_1' => true,
+                    'test_key_2' => false,
+                ],
+            ],
             'Test mapping getting information from the context' => [
                 'mappings' => [
                     'x_to_xyz' => [
@@ -178,6 +195,40 @@ public function testMapForEmptyMappingName()
         );
     }
 
+    /**
+     * @covers ::map
+     */
+    public function testKeyExistsForExceptions()
+    {
+        $this->expectException(\InvalidArgumentException::class);
+        $this->mapper->addMappings([
+            'mapping_name' => [
+                'test_key_1' => "mapper.keyExists(obj.get('test_array'), obj.get('test_key'))",
+            ],
+        ]);
+
+        // Testing 1st argument
+        $this->expectExceptionMessage('keyExists expected the first argument to be an array');
+        $this->mapper->map(
+            new SerializableArray([
+                'test_array' => 'A',
+                'test_key' => 'B',
+            ]),
+            'mapping_name'
+        );
+
+        // Testing 2nd argument
+        $this->expectExceptionMessage('keyExists expected the key (second argument) to be either a string or an integer');
+        $this->mapper->map(
+            new SerializableArray([
+                'test_array' => ['A' => 0, 'B' => 0],
+                'test_key' => [10],
+            ]),
+            'mapping_name'
+        );
+
+    }
+
     /**
      * @covers ::map
      */
diff --git a/Tools/Mapper/Mapper.php b/Tools/Mapper/Mapper.php
index 810b43c6..66e1ebaa 100644
--- a/Tools/Mapper/Mapper.php
+++ b/Tools/Mapper/Mapper.php
@@ -42,7 +42,7 @@ public function addMappings(array $mappings)
      */
     public function formatDate($format, \DateTime $date = null)
     {
-        if ($date === null) {
+        if (null === $date) {
             return null;
         }
 
@@ -87,7 +87,7 @@ public function resolve(&$mapping, &$obj, &$context)
             foreach ($mapping as $key => $value) {
                 $resolved = $this->resolve($value, $obj, $context);
 
-                if ($resolved !== null) {
+                if (null !== $resolved) {
                     $res[$key] = $resolved;
                 }
             }
@@ -144,9 +144,17 @@ public function mapAll($elements, $mappingName, $context = [])
      *
      * @return bool
      */
-    public function keyExists(array $obj, $key)
+    public function keyExists($array, $key)
     {
-        return array_key_exists($key, $obj);
+        if (!is_array($array)) {
+            throw new \InvalidArgumentException('keyExists expected the first argument to be an array, \''.print_r($array, true).'\' was given.');
+        }
+
+        if (!is_string($key) and !is_integer($key)) {
+            throw new \InvalidArgumentException('keyExists expected the key (second argument) to be either a string or an integer, \''.print_r($key, true).'\' was given.');
+        }
+
+        return array_key_exists($key, $array);
     }
 
     /**
@@ -302,11 +310,11 @@ public function serializeWithGroup($data, $format, $group)
     }
 
     /**
-     * Return the n-th section of the given string splitted by piece of the given length
+     * Return the n-th section of the given string splitted by piece of the given length.
      *
      * @param string $string
-     * @param int $length
-     * @param int $section
+     * @param int    $length
+     * @param int    $section
      *
      * @return string
      */
@@ -320,6 +328,7 @@ public function wordWrap($string, $length, $section)
         if (isset($lines[$section])) {
             $result = $lines[$section];
         }
+
         return $result;
     }