diff --git a/typo3/sysext/backend/Resources/Public/JavaScript/FormEngineValidation.js b/typo3/sysext/backend/Resources/Public/JavaScript/FormEngineValidation.js
index 5d51a019ac64336f3c43df44cb40f901cc1fdece..894d50c145a7883e3b6417d594889bdfd2eb9df2 100644
--- a/typo3/sysext/backend/Resources/Public/JavaScript/FormEngineValidation.js
+++ b/typo3/sysext/backend/Resources/Public/JavaScript/FormEngineValidation.js
@@ -429,7 +429,7 @@ define([
       case 'alphanum_x':
         newString = '';
         for (a = 0; a < value.length; a++) {
-          theChar = value.substr(a, 1);
+          var theChar = value.substr(a, 1);
           var special = (theChar === '_' || theChar === '-');
           var alpha = (theChar >= 'a' && theChar <= 'z') || (theChar >= 'A' && theChar <= 'Z');
           var num = (theChar >= '0' && theChar <= '9');