diff --git a/.travis.yml b/.travis.yml
index d850c56..36a8175 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -28,7 +28,7 @@ script:
   # Run with phpdbg or xdebug as soon as one of both doesn't segfault anymore
   - vendor/bin/phpunit --coverage-text --coverage-clover build/logs/clover.xml
   - (for f in examples/*.php; do echo $f && if ! php $f; then echo '!! failed !!' && exit 1; fi && echo "-------"; done)
-  - PHP_CS_FIXER_IGNORE_ENV=1 php vendor/bin/php-cs-fixer --diff --dry-run -v fix
+  - PHP_CS_FIXER_IGNORE_ENV=1 composer fix-syntax -- --dry-run
 
 after_script:
   - ./coveralls.phar -v
diff --git a/composer.json b/composer.json
index a34c517..b06ee53 100644
--- a/composer.json
+++ b/composer.json
@@ -17,6 +17,11 @@
             "src/functions.php"
         ]
     },
+    "autoload-dev": {
+        "psr-4": {
+            "Amp\\ParallelFunctions\\Test\\": "test"
+        }
+    },
     "require": {
         "php": ">=7",
         "amphp/parallel": "^0.1.8 || ^0.2",
@@ -32,5 +37,8 @@
         "platform": {
             "php": "7.0.0"
         }
+    },
+    "scripts": {
+        "fix-syntax": "php vendor/bin/php-cs-fixer --diff -v fix"
     }
 }
diff --git a/test/FilterTest.php b/test/FilterTest.php
index 03904f7..8a0edd1 100644
--- a/test/FilterTest.php
+++ b/test/FilterTest.php
@@ -74,4 +74,16 @@ class FilterTest extends TestCase {
             $this->assertStringEqualsFile($files[2][1], '2');
         }
     }
+
+    public function testFilterWithNullCallable() {
+        $this->expectException(\Error::class);
+
+        $files = [
+            [0, \tempnam(\sys_get_temp_dir(), 'amp-parallel-functions-')],
+            [1, \tempnam(\sys_get_temp_dir(), 'amp-parallel-functions-')],
+            [2, \tempnam(\sys_get_temp_dir(), 'amp-parallel-functions-')],
+        ];
+
+        wait(parallelFilter($files, null, ARRAY_FILTER_USE_BOTH));
+    }
 }