diff --git a/composer.json b/composer.json
index 9dbd149..df6faed 100644
--- a/composer.json
+++ b/composer.json
@@ -55,6 +55,6 @@
         "php": ">=5.3.3"
     },
     "require-dev": {
-        "phpunit/phpunit": "4.8.*"
+        "phpunit/phpunit": "<6.0"
     }
 }