diff --git a/composer.json b/composer.json
index 3e5ebd9..a45b2f5 100644
--- a/composer.json
+++ b/composer.json
@@ -15,7 +15,8 @@
     }
   ],
   "require": {
-    "php": ">=5.5"
+    "php": ">=5.5",
+    "ext-json": "*"
   },
   "require-dev": {
     "phpunit/phpunit": "4.4.*"