diff --git a/composer.json b/composer.json
index 347c233..5c3925c 100644
--- a/composer.json
+++ b/composer.json
@@ -8,7 +8,7 @@
     "guzzlehttp/guzzle": ">=6.2.2"
   },
   "require-dev": {
-    "phpunit/phpunit": ">=4.8.12",
+    "phpunit/phpunit": "4.8.12",
     "satooshi/php-coveralls": "dev-master"
   },
   "autoload": {