diff --git a/composer.json b/composer.json
index 0d5bef6..c6c65ba 100644
--- a/composer.json
+++ b/composer.json
@@ -5,7 +5,7 @@
         ]
     },
     "require-dev": {
-        "phpunit/phpunit": "*",
+        "phpunit/phpunit": "5.7",
         "phpdocumentor/phpdocumentor": "2.*",
         "phpmd/phpmd": "^2.6",
         "gamegos/php-code-sniffer": "*"