diff --git a/composer.json b/composer.json
index dc349d4..f8b40f8 100644
--- a/composer.json
+++ b/composer.json
@@ -13,7 +13,8 @@
         }
     ],
     "require": {
-        "php": ">=5.3.0"
+        "php": ">=5.3.0",
+        "ext-mbstring": "*"
     },
     "require-dev": {
         "phpunit/phpunit": "^4.8.35"