diff --git a/src/View/StringTemplate.php b/src/View/StringTemplate.php
index da724bb11be..05d296fa008 100644
--- a/src/View/StringTemplate.php
+++ b/src/View/StringTemplate.php
@@ -328,15 +328,9 @@ protected function _formatAttribute($key, $value, $escape = true)
     /**
      * Adds a class and returns a unique list either in array or space separated
      *
-     * ### Options
-     *
-     * - `useIndex` if you are inputting an array with an 'element' other than 'class'.
-     *              Also setting to 'false' will manipulate the whole array (default is 'class')
-     *
      * @param array|string $input The array or string to add the class to
      * @param array|string $newClass the new class or classes to add
-     * @param string $useIndex if you are inputting an array with an 'element' other than 'class'.
-     *               Also setting to 'false' will manipulate the whole array (default is 'class')
+     * @param string $useIndex if you are inputting an array with an element other than default of 'class'.
      * @return array|string
      */
     public function addClass($input, $newClass, $useIndex = 'class')
@@ -346,10 +340,11 @@ public function addClass($input, $newClass, $useIndex = 'class')
             return $input;
         }
 
-        if (is_string($useIndex) && is_array($input)) {
+        if (is_array($input)) {
             $class = Hash::get($input, $useIndex, []);
         } else {
             $class = $input;
+            $input = [];
         }
 
         // Convert and sanitise the inputs
@@ -367,14 +362,7 @@ public function addClass($input, $newClass, $useIndex = 'class')
 
         $class = array_unique(array_merge($class, $newClass));
 
-        if (is_string($useIndex)) {
-            if (!is_array($input)) {
-                $input = [];
-            }
-            $input = Hash::insert($input, $useIndex, $class);
-        } else {
-            $input = $class;
-        }
+        $input = Hash::insert($input, $useIndex, $class);
 
         return $input;
     }
diff --git a/tests/TestCase/View/StringTemplateTest.php b/tests/TestCase/View/StringTemplateTest.php
index 96eba99033b..f59349d0eaa 100644
--- a/tests/TestCase/View/StringTemplateTest.php
+++ b/tests/TestCase/View/StringTemplateTest.php
@@ -397,16 +397,20 @@ public function testAddClassMethodUseIndex()
 
         $result = $this->template->addClass(
             [
-                'current_class',
-                'text'
+                'class' => [
+                    'current_class',
+                    'text'
+                ]
             ],
             'new_class',
-            false
+            'non-existent'
         );
         $this->assertEquals($result, [
-            'current_class',
-            'text',
-            'new_class'
+            'class' => [
+                'current_class',
+                'text'
+            ],
+            'non-existent' => ['new_class']
         ]);
     }
 }