diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index e2f84ee67..9d271a98c 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -15,7 +15,7 @@ jobs:
 
         runs-on: 'ubuntu-latest'
 
-        continue-on-error: true
+        continue-on-error: ${{ matrix.experimental }}
 
         strategy:
             matrix:
@@ -24,6 +24,10 @@ jobs:
                     - '7.3'
                     - '7.4'
                     - '8.0'
+                composer-options: ['']
+                experimental: [false]
+                include:
+                  - { php-version: '8.1', experimental: true, composer-options: '--ignore-platform-req=php' }
 
         steps:
             - name: "Checkout code"
@@ -51,7 +55,7 @@ jobs:
                   key: ${{ runner.os }}-${{ matrix.php-version }}-composer-${{ hashFiles('composer.json') }}
                   restore-keys: ${{ runner.os }}-${{ matrix.php-version }}-composer-
 
-            - run: composer install
+            - run: composer install ${{ matrix.composer-options }}
 
             - name: "Install PHPUnit"
               run: vendor/bin/simple-phpunit install
diff --git a/src/Environment.php b/src/Environment.php
index d25208d60..e3dfaa167 100644
--- a/src/Environment.php
+++ b/src/Environment.php
@@ -623,7 +623,7 @@ class Environment
      */
     public function setCharset($charset)
     {
-        if ('UTF8' === $charset = strtoupper($charset)) {
+        if ('UTF8' === $charset = null === $charset ? null : strtoupper($charset)) {
             // iconv on Windows requires "UTF-8" instead of "UTF8"
             $charset = 'UTF-8';
         }
diff --git a/src/Error/Error.php b/src/Error/Error.php
index 3260b4895..21e10e632 100644
--- a/src/Error/Error.php
+++ b/src/Error/Error.php
@@ -200,7 +200,7 @@ class Error extends \Exception
         foreach ($backtrace as $trace) {
             if (isset($trace['object']) && $trace['object'] instanceof Template && 'Twig\Template' !== \get_class($trace['object'])) {
                 $currentClass = \get_class($trace['object']);
-                $isEmbedContainer = 0 === strpos($templateClass, $currentClass);
+                $isEmbedContainer = null === $templateClass ? false : 0 === strpos($templateClass, $currentClass);
                 if (null === $this->name || ($this->name == $trace['object']->getTemplateName() && !$isEmbedContainer)) {
                     $template = $trace['object'];
                     $templateClass = \get_class($trace['object']);
diff --git a/src/Extension/CoreExtension.php b/src/Extension/CoreExtension.php
index 0ebef7f1c..29fbc3a23 100644
--- a/src/Extension/CoreExtension.php
+++ b/src/Extension/CoreExtension.php
@@ -519,6 +519,9 @@ function twig_date_converter(Environment $env, $date = null, $timezone = null)
     }
 
     if (null === $date || 'now' === $date) {
+        if ($date === null) {
+            $date = 'now';
+        }
         return new \DateTime($date, false !== $timezone ? $timezone : $env->getExtension(CoreExtension::class)->getTimezone());
     }
 
diff --git a/src/Loader/FilesystemLoader.php b/src/Loader/FilesystemLoader.php
index 0971e09b5..6dfa67426 100644
--- a/src/Loader/FilesystemLoader.php
+++ b/src/Loader/FilesystemLoader.php
@@ -37,7 +37,7 @@ class FilesystemLoader implements LoaderInterface, ExistsLoaderInterface, Source
     public function __construct($paths = [], string $rootPath = null)
     {
         $this->rootPath = (null === $rootPath ? getcwd() : $rootPath).\DIRECTORY_SEPARATOR;
-        if (false !== $realPath = realpath($rootPath)) {
+        if ($rootPath !== null && false !== ($realPath = realpath($rootPath))) {
             $this->rootPath = $realPath.\DIRECTORY_SEPARATOR;
         }