diff --git a/composer.json b/composer.json
index 723596e..326b684 100644
--- a/composer.json
+++ b/composer.json
@@ -31,7 +31,8 @@
         "sebastian/recursion-context": "~1.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "~4.4"
+        "phpunit/phpunit": "~4.4",
+        "ext-mbstring": "*"
     },
     "autoload": {
         "classmap": [