diff --git a/composer.json b/composer.json
index 076d5ff..4c222a1 100644
--- a/composer.json
+++ b/composer.json
@@ -7,7 +7,7 @@
     },
     "require-dev": {
         "phpspec/phpspec": "^3.4",
-        "phpunit/phpunit": "^5",
+        "phpunit/phpunit": "~5.5",
         "aws/aws-sdk-php": "~3"
     },
     "license": "MIT",