diff --git a/composer.json b/composer.json
index c9128ce..ba079f1 100644
--- a/composer.json
+++ b/composer.json
@@ -9,7 +9,8 @@
     "API"
   ],
   "require": {
-    "php": ">=5.4,<8.0-DEV"
+    "php": ">=5.4,<8.0-DEV",
+    "ext-mbstring": "*"
   },
   "require-dev": {
     "phpunit/phpunit": "^4.8"