diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index a63a5ead6..0ad9c6328 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -19,6 +19,7 @@ jobs:
         php-version:
           - '7.4'
           - '8.0'
+          - '8.1'
         symfony-version:
           - '4.4.*'
           - '5.3.*'
diff --git a/composer.json b/composer.json
index 87e1413b8..5e6e5ade2 100644
--- a/composer.json
+++ b/composer.json
@@ -59,7 +59,7 @@
         "symfony/console": "^4.4 || ^5.3",
         "symfony/css-selector": "^4.4 || ^5.3",
         "symfony/monolog-bundle": "^3.7",
-        "symfony/phpunit-bridge": "^4.4 || ^5.3",
+        "symfony/phpunit-bridge": "^6.0",
         "symfony/process": "^4.4 || ^5.3",
         "symfony/security-bundle": "^4.4 || ^5.3",
         "symfony/validator": "^4.4 || ^5.3",
@@ -69,8 +69,7 @@
     },
     "conflict": {
         "phpdocumentor/type-resolver": "<1.4.0",
-        "react/promise": "<2.8",
-        "symfony/phpunit-bridge": "<4.4.9"
+        "react/promise": "<2.8"
     },
     "extra": {
         "branch-alias": {
diff --git a/src/Definition/Argument.php b/src/Definition/Argument.php
index 2cf8fa815..6fd71215c 100644
--- a/src/Definition/Argument.php
+++ b/src/Definition/Argument.php
@@ -42,8 +42,11 @@ public function offsetExists($offset): bool
      *
      * @return mixed|null
      */
+    #[\ReturnTypeWillChange]
     public function offsetGet($offset)
     {
+        // TODO 1.0: Drop PHP 7.4 support and add mixed return type.
+
         return $this->offsetExists($offset) ? $this->rawArguments[$offset] : null;
     }