diff --git a/composer.json b/composer.json
index 34a9c47..2bc64c1 100644
--- a/composer.json
+++ b/composer.json
@@ -20,7 +20,7 @@
         "php" : "~5.6|~7.0"
     },
     "require-dev": {
-        "phpunit/phpunit" : "~4.0||~5.0",
+        "phpunit/phpunit" : "~4.0||~5.0||~6.0",
         "squizlabs/php_codesniffer": "^2.3"
     },
     "autoload": {