diff --git a/src/LanguageLoader.php b/src/LanguageLoader.php
index 245132a..fc97e02 100644
--- a/src/LanguageLoader.php
+++ b/src/LanguageLoader.php
@@ -147,7 +147,7 @@ protected static function operatorForWhere($key, $operator, $value)
      *
      * @return array
      */
-    protected static function filter($items, callable $callback = null)
+    protected static function filter($items, ?callable $callback = null)
     {
         if ($callback) {
             return array_filter($items, $callback, ARRAY_FILTER_USE_BOTH);
diff --git a/tests/Unit/LanguageLoaderTest.php b/tests/Unit/LanguageLoaderTest.php
index d7b1b6d..a7c8059 100644
--- a/tests/Unit/LanguageLoaderTest.php
+++ b/tests/Unit/LanguageLoaderTest.php
@@ -308,7 +308,8 @@ public function it_collapses_array()
     /** @test */
     public function it_gets_file_content()
     {
-        $this->assertStringEqualsFile(__DIR__.'/../resources/languages.json', self::$methods['getFile']->invoke(null, __DIR__.'/../resources/languages.json'));
+        $path = realpath(dirname(__DIR__, 2).'/resources/languages.json');
+        $this->assertStringEqualsFile($path, self::$methods['getFile']->invoke(null, $path));
     }
 
     /** @test */