diff --git a/composer.json b/composer.json
index 6d561be97..ed048b091 100644
--- a/composer.json
+++ b/composer.json
@@ -49,7 +49,8 @@
         "ext-session": "*",
         "psr/container-implementation": "1.0.0",
         "psr/container": "1.0.0",
-        "silverstripe/vendor-plugin": "^1.0"
+        "silverstripe/vendor-plugin": "^1.0",
+        "ext-json": "*"
     },
     "require-dev": {
         "phpunit/phpunit": "^5.7",