diff --git a/composer.json b/composer.json
index 53432ee1..33360af1 100644
--- a/composer.json
+++ b/composer.json
@@ -40,7 +40,7 @@
         "ext-json": "*"
     },
     "require-dev": {
-	    	"phpunit/phpunit"	    : "^6",
+		"phpunit/phpunit"	    : "6.5.13",
 		"pdepend/pdepend"           : "2.5.0",
 		"phpmd/phpmd"               : "2.6.0",
 		"sebastian/phpcpd"          : "3.0.1",