diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 6fb7a11..82be3a0 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -23,7 +23,7 @@ jobs:
         include:
           - php-version: '8.1'
             dependencies: 'lowest'
-          - php-version: '8.3'
+          - php-version: '8.4'
             dependencies: 'highest'
 
     steps:
@@ -67,7 +67,7 @@ jobs:
       - name: Setup PHP
         uses: shivammathur/setup-php@v2
         with:
-          php-version: '8.1'
+          php-version: '8.4'
           extensions: mbstring, intl
           coverage: none
           tools: cs2pr
diff --git a/composer.json b/composer.json
index 1ce851e..d48beba 100644
--- a/composer.json
+++ b/composer.json
@@ -16,7 +16,7 @@
     },
     "require-dev": {
         "cakephp/cakephp-codesniffer": "^5.0",
-        "phpunit/phpunit": "^10.5.5 || ^11.1.3 || ^12.0"
+        "phpunit/phpunit": "^10.5.5 || ^11.1.3 || ^12.0 || ^13.0"
     },
     "autoload": {
         "psr-4": {
diff --git a/src/TagEngine.php b/src/TagEngine.php
index 1beaac5..8f649d3 100644
--- a/src/TagEngine.php
+++ b/src/TagEngine.php
@@ -154,6 +154,7 @@ public function parse(mixed $source = false, array $data = []): string
         if ($result === null) {
             $this->throwOnNull($source);
         }
+        assert(is_string($result));
 
         return $result;
     }