diff --git a/composer.json b/composer.json
index 1948004..a068b8c 100644
--- a/composer.json
+++ b/composer.json
@@ -9,7 +9,7 @@
     "paragonie/sodium_compat": "^1.13"
   },
   "require-dev": {
-    "phpunit/phpunit": "^7|^8|^9",
+    "phpunit/phpunit": "^5|^6|^7|^8|^9",
     "vimeo/psalm": "^3|^4"
   },
   "autoload": {