diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index f9e05c03..ecd5af1a 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -13,7 +13,7 @@ jobs:
             fail-fast: false
             matrix:
                 os: [ubuntu-latest]
-                php: ['7.2', '7.3', '7.4', '8.0']
+                php: ['7.2', '7.3', '7.4', '8.0', '8.1']
 
         steps:
             - name: Checkout
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8f473f6a..1c754680 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,7 @@ Yii Framework 2 apidoc extension Change Log
 -----------------------
 
 - Bug #313: Fix deprecation error `Method deprecated, use ::getParameters()` (mspirkov)
+- Bug #317: Fix `trim` deprecation errors `Passing null to parameter #1 ($string) of type string is deprecated` (mspirkov)
 
 
 3.0.7 February 13, 2025
diff --git a/commands/ApiController.php b/commands/ApiController.php
index bd7d6425..b584b2ed 100644
--- a/commands/ApiController.php
+++ b/commands/ApiController.php
@@ -31,7 +31,7 @@ class ApiController extends BaseController
      */
     public $guidePrefix = 'guide-';
     /**
-     * @var string Repository url (e.g. "https://github.com/yiisoft/yii2"). Optional, used for resolving relative links
+     * @var string|null Repository url (e.g. "https://github.com/yiisoft/yii2"). Optional, used for resolving relative links
      * within a repository (e.g. "[docs/guide/README.md](docs/guide/README.md)"). If you don't have such links you can
      * skip this. Otherwise, skipping this will cause generation of broken links because they will be not resolved and
      * left as is.
@@ -74,7 +74,7 @@ public function actionIndex(array $sourceDirs, $targetDir)
             }
         }
 
-        $renderer->repoUrl = rtrim($this->repoUrl, '/');
+        $renderer->repoUrl = $this->repoUrl !== null ? rtrim($this->repoUrl, '/') : null;
 
         // search for files to process
         if (($files = $this->searchFiles($sourceDirs)) === false) {
diff --git a/composer.json b/composer.json
index 0f8a2577..bac01dc7 100644
--- a/composer.json
+++ b/composer.json
@@ -32,7 +32,7 @@
         "scrivo/highlight.php": "^9.0"
     },
     "require-dev": {
-        "phpunit/phpunit": "*"
+        "phpunit/phpunit": "^8.5"
     },
     "repositories": [
         {
diff --git a/models/ClassDoc.php b/models/ClassDoc.php
index 4916c69f..c68cb1c4 100644
--- a/models/ClassDoc.php
+++ b/models/ClassDoc.php
@@ -18,7 +18,7 @@
 class ClassDoc extends TypeDoc
 {
     /**
-     * @var string
+     * @var string|null
      */
     public $parentClass;
     /**
@@ -101,7 +101,8 @@ public function __construct($reflector = null, $context = null, $config = [])
             return;
         }
 
-        $this->parentClass = ltrim($reflector->getParent(), '\\');
+        $reflectorParent = $reflector->getParent();
+        $this->parentClass = $reflectorParent !== null ? ltrim($reflectorParent, '\\') : null;
         if (empty($this->parentClass)) {
             $this->parentClass = null;
         }