diff --git a/.github/workflows/php-tests.yml b/.github/workflows/php-tests.yml
index 5f8e683..a10f73c 100644
--- a/.github/workflows/php-tests.yml
+++ b/.github/workflows/php-tests.yml
@@ -9,7 +9,7 @@ jobs:
   php-tests:
     uses: lunr-php/actions-templates/.github/workflows/php-composer.yml@master
     with:
-      phpstan-level: 7
+      phpstan-level: 10
       allow-style-failures: false
       allow-phpstan-failures: false
       codestyle-branch: '0.10.1'
diff --git a/src/Lunr/Core/Configuration.php b/src/Lunr/Core/Configuration.php
index 2184686..a98b773 100644
--- a/src/Lunr/Core/Configuration.php
+++ b/src/Lunr/Core/Configuration.php
@@ -311,9 +311,9 @@ public function current(): mixed
      *
      * (Inherited from Iterator)
      *
-     * @return scalar Scalar on success, NULL on failure
+     * @return array-key|null Value on success, NULL on failure
      */
-    public function key(): mixed
+    public function key(): int|string|null
     {
         return key($this->config);
     }