diff --git a/composer.json b/composer.json
index 3b14ebe..1579358 100644
--- a/composer.json
+++ b/composer.json
@@ -30,6 +30,6 @@
     "ext-sockets": "*"
   },
   "require-dev": {
-    "phpunit/phpunit": "^7.0"
+    "phpunit/phpunit": "^6.0"
   }
 }