diff --git a/composer.json b/composer.json
index 6e26cd3..69a3463 100644
--- a/composer.json
+++ b/composer.json
@@ -13,7 +13,8 @@
         "guzzlehttp/guzzle": "~6.0",
         "PHP": "^7.1",
         "kornrunner/keccak": "~1.0",
-        "phpseclib/phpseclib": "~2.0.11"
+        "phpseclib/phpseclib": "~2.0.11",
+        "ext-mbstring": "*"
     },
     "require-dev": {
         "phpunit/phpunit": "~6.0"