diff --git a/composer.json b/composer.json
index dc8b6a3..9157123 100644
--- a/composer.json
+++ b/composer.json
@@ -31,7 +31,8 @@
     "friendsofphp/php-cs-fixer": "^3.16",
     "phpstan/phpstan": "^1.10",
     "rector/rector": "^1.0",
-    "symfony/finder": "^5.4"
+    "symfony/finder": "^5.4",
+    "phpstan/phpstan-strict-rules": "^1.5"
   },
   "autoload": {
     "psr-4": {
diff --git a/composer.lock b/composer.lock
index 430b42d..20211a9 100644
--- a/composer.lock
+++ b/composer.lock
@@ -4,7 +4,7 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies",
         "This file is @generated automatically"
     ],
-    "content-hash": "d4d41a03a213cbbfadc482a12285b9e3",
+    "content-hash": "5fbea03cea42981c9095d287f95a4f18",
     "packages": [
         {
             "name": "doctrine/deprecations",
@@ -1089,6 +1089,55 @@
             ],
             "time": "2024-02-20T13:59:13+00:00"
         },
+        {
+            "name": "phpstan/phpstan-strict-rules",
+            "version": "1.5.2",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/phpstan/phpstan-strict-rules.git",
+                "reference": "7a50e9662ee9f3942e4aaaf3d603653f60282542"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/phpstan/phpstan-strict-rules/zipball/7a50e9662ee9f3942e4aaaf3d603653f60282542",
+                "reference": "7a50e9662ee9f3942e4aaaf3d603653f60282542",
+                "shasum": ""
+            },
+            "require": {
+                "php": "^7.2 || ^8.0",
+                "phpstan/phpstan": "^1.10.34"
+            },
+            "require-dev": {
+                "nikic/php-parser": "^4.13.0",
+                "php-parallel-lint/php-parallel-lint": "^1.2",
+                "phpstan/phpstan-deprecation-rules": "^1.1",
+                "phpstan/phpstan-phpunit": "^1.0",
+                "phpunit/phpunit": "^9.5"
+            },
+            "type": "phpstan-extension",
+            "extra": {
+                "phpstan": {
+                    "includes": [
+                        "rules.neon"
+                    ]
+                }
+            },
+            "autoload": {
+                "psr-4": {
+                    "PHPStan\\": "src/"
+                }
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "MIT"
+            ],
+            "description": "Extra strict and opinionated rules for PHPStan",
+            "support": {
+                "issues": "https://github.com/phpstan/phpstan-strict-rules/issues",
+                "source": "https://github.com/phpstan/phpstan-strict-rules/tree/1.5.2"
+            },
+            "time": "2023-10-30T14:35:06+00:00"
+        },
         {
             "name": "phpunit/php-code-coverage",
             "version": "9.2.26",
diff --git a/phpstan.neon b/phpstan.neon
index 166988e..12eae68 100644
--- a/phpstan.neon
+++ b/phpstan.neon
@@ -1,3 +1,6 @@
+includes:
+    - vendor/phpstan/phpstan-strict-rules/rules.neon
+
 parameters:
     level: max
     paths:
diff --git a/tests/zugferd2/Tests/Legacy/TradeAccountingAccountTest.php b/tests/zugferd2/Tests/Legacy/TradeAccountingAccountTest.php
index 03dc554..f8fe5c2 100644
--- a/tests/zugferd2/Tests/Legacy/TradeAccountingAccountTest.php
+++ b/tests/zugferd2/Tests/Legacy/TradeAccountingAccountTest.php
@@ -108,7 +108,7 @@ public function testTradeAccountingAccount(): void
   </rsm:SupplyChainTradeTransaction>
 </rsm:CrossIndustryInvoice>
 XML;
-        $this->assertEquals(
+        self::assertEquals(
             // Removes white-space
             preg_replace('/\s/', '', $xml),
             preg_replace('/\s/', '', Builder::create()->transform($invoice))