diff --git a/composer.json b/composer.json
index b4d5ea0..6bad87f 100644
--- a/composer.json
+++ b/composer.json
@@ -18,7 +18,7 @@
         "symfony/options-resolver": "^3.4|^4.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "^7.5",
+        "phpunit/phpunit": "^5.7",
         "php-http/guzzle6-adapter": "^1.1"
     },
     "autoload": {