diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 465821a..ec875c8 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -7,7 +7,7 @@ on:
 jobs:
   PHPUnit:
     name: PHPUnit (PHP ${{ matrix.php }})
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-22.04
     strategy:
       matrix:
         php:
@@ -23,11 +23,12 @@ jobs:
           - 5.4
           - 5.3
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - uses: shivammathur/setup-php@v2
         with:
           php-version: ${{ matrix.php }}
           coverage: xdebug
+          ini-file: development
       - run: composer install
       - run: vendor/bin/phpunit --coverage-text
         if: ${{ matrix.php >= 7.3 }}
diff --git a/README.md b/README.md
index 3da9fb3..641750f 100644
--- a/README.md
+++ b/README.md
@@ -1,6 +1,6 @@
 # clue/reactphp-mq
 
-[![CI status](https://github.com/clue/reactphp-mq/workflows/CI/badge.svg)](https://github.com/clue/reactphp-mq/actions)
+[![CI status](https://github.com/clue/reactphp-mq/actions/workflows/ci.yml/badge.svg)](https://github.com/clue/reactphp-mq/actions)
 [![installs on Packagist](https://img.shields.io/packagist/dt/clue/mq-react?color=blue&label=installs%20on%20Packagist)](https://packagist.org/packages/clue/mq-react)
 
 Mini Queue, the lightweight in-memory message queue to concurrently do many (but not too many) things at once,
@@ -485,7 +485,7 @@ This project follows [SemVer](https://semver.org/).
 This will install the latest supported version:
 
 ```bash
-$ composer require clue/mq-react:^1.5
+composer require clue/mq-react:^1.5
 ```
 
 See also the [CHANGELOG](CHANGELOG.md) for details about version upgrades.
@@ -500,13 +500,13 @@ To run the test suite, you first need to clone this repo and then install all
 dependencies [through Composer](https://getcomposer.org/):
 
 ```bash
-$ composer install
+composer install
 ```
 
 To run the test suite, go to the project root and run:
 
 ```bash
-$ vendor/bin/phpunit
+vendor/bin/phpunit
 ```
 
 ## License
diff --git a/composer.json b/composer.json
index 88ab706..8a83e34 100644
--- a/composer.json
+++ b/composer.json
@@ -10,20 +10,24 @@
             "email": "christian@clue.engineering"
         }
     ],
-    "autoload": {
-        "psr-4": { "Clue\\React\\Mq\\": "src/" }
-    },
-    "autoload-dev": {
-        "psr-4": { "Clue\\Tests\\React\\Mq\\": "tests/" }
-    },
     "require": {
         "php": ">=5.3",
         "react/promise": "^3 || ^2.2.1 || ^1.2.1"
     },
     "require-dev": {
-        "phpunit/phpunit": "^9.3 || ^5.7 || ^4.8.35",
+        "phpunit/phpunit": "^9.6 || ^5.7 || ^4.8.36",
         "react/async": "^4 || ^3 || ^2",
         "react/event-loop": "^1.2",
         "react/http": "^1.8"
+    },
+    "autoload": {
+        "psr-4": {
+            "Clue\\React\\Mq\\": "src/"
+        }
+    },
+    "autoload-dev": {
+        "psr-4": {
+            "Clue\\Tests\\React\\Mq\\": "tests/"
+        }
     }
 }
diff --git a/phpunit.xml.dist b/phpunit.xml.dist
index befe85d..93fd66f 100644
--- a/phpunit.xml.dist
+++ b/phpunit.xml.dist
@@ -1,14 +1,14 @@
 <?xml version="1.0" encoding="UTF-8"?>
 
-<!-- PHPUnit configuration file with new format for PHPUnit 9.3+ -->
-<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
-         xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/9.3/phpunit.xsd"
+<!-- PHPUnit configuration file with new format for PHPUnit 9.6+ -->
+<phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/9.6/phpunit.xsd"
          bootstrap="vendor/autoload.php"
-         colors="true"
          cacheResult="false"
+         colors="true"
          convertDeprecationsToExceptions="true">
     <testsuites>
-        <testsuite name="MQ React Test Suite">
+        <testsuite name="MQ Test Suite">
             <directory>./tests/</directory>
         </testsuite>
     </testsuites>
@@ -17,4 +17,12 @@
             <directory>./src/</directory>
         </include>
     </coverage>
+    <php>
+        <ini name="error_reporting" value="-1" />
+        <!-- Evaluate assertions, requires running with "php -d zend.assertions=1 vendor/bin/phpunit" -->
+        <!-- <ini name="zend.assertions" value="1" /> -->
+        <ini name="assert.active" value="1" />
+        <ini name="assert.exception" value="1" />
+        <ini name="assert.bail" value="0" />
+    </php>
 </phpunit>
diff --git a/phpunit.xml.legacy b/phpunit.xml.legacy
index 47cbb40..deaf1bc 100644
--- a/phpunit.xml.legacy
+++ b/phpunit.xml.legacy
@@ -1,13 +1,12 @@
 <?xml version="1.0" encoding="UTF-8"?>
 
-<!-- PHPUnit configuration file with old format for PHPUnit 9.2 or older -->
+<!-- PHPUnit configuration file with old format for legacy PHPUnit -->
 <phpunit xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:noNamespaceSchemaLocation="https://schema.phpunit.de/4.8/phpunit.xsd"
          bootstrap="vendor/autoload.php"
-         colors="true"
-         convertDeprecationsToExceptions="true">
+         colors="true">
     <testsuites>
-        <testsuite name="MQ React Test Suite">
+        <testsuite name="MQ Test Suite">
             <directory>./tests/</directory>
         </testsuite>
     </testsuites>
@@ -16,4 +15,12 @@
             <directory>./src/</directory>
         </whitelist>
     </filter>
+    <php>
+        <ini name="error_reporting" value="-1" />
+        <!-- Evaluate assertions, requires running with "php -d zend.assertions=1 vendor/bin/phpunit" -->
+        <!-- <ini name="zend.assertions" value="1" /> -->
+        <ini name="assert.active" value="1" />
+        <ini name="assert.exception" value="1" />
+        <ini name="assert.bail" value="0" />
+    </php>
 </phpunit>
diff --git a/src/Queue.php b/src/Queue.php
index f4e827b..29357d6 100644
--- a/src/Queue.php
+++ b/src/Queue.php
@@ -363,6 +363,7 @@ public function __invoke()
         $queue[] = null;
         end($queue);
         $id = key($queue);
+        assert(is_int($id));
 
         $deferred = new Deferred(function ($_, $reject) use (&$queue, $id, &$deferred) {
             // forward cancellation to pending operation if it is currently executing
@@ -420,8 +421,8 @@ public function processQueue()
             return;
         }
 
-        /* @var $deferred Deferred */
         $deferred = reset($this->queue);
+        assert($deferred instanceof Deferred);
         unset($this->queue[key($this->queue)]);
 
         // once number of pending jobs is below concurrency limit again: