diff --git a/src/StringUtil.php b/src/StringUtil.php
index 4fa1613..b78b200 100644
--- a/src/StringUtil.php
+++ b/src/StringUtil.php
@@ -46,10 +46,8 @@ private static function findCommonPrefix(string $string1, string $string2): stri
             if (!isset($string2[$i]) || $string1[$i] != $string2[$i]) {
                 break;
             }
-
-            $prefix .= $string1[$i];
         }
 
-        return $prefix;
+        return substr($string1, 0, $i);
     }
 }