diff --git a/composer.json b/composer.json
index 98b5f2b..572251a 100644
--- a/composer.json
+++ b/composer.json
@@ -15,7 +15,8 @@
         }
     ],
     "require": {
-        "php": ">=5.3.0"
+        "php": ">=5.3.0",
+        "symfony/polyfill-mbstring": "^1.1"
     },
     "require-dev": {
         "phpunit/phpunit": "~4.0"