diff --git a/composer.json b/composer.json
index 15ac609..12ccfee 100644
--- a/composer.json
+++ b/composer.json
@@ -32,7 +32,7 @@
     "require-dev": {
         "amphp/amp": "^2.4",
         "amphp/parallel": "^1.3",
-        "phpunit/phpunit": "^9",
+        "phpunit/phpunit": "^7 | ^8 | ^9",
         "amphp/phpunit-util": "^1.3",
         "amphp/php-cs-fixer-config": "dev-master"
     },