diff --git a/classes/LocaleMiddleware.php b/classes/LocaleMiddleware.php
index 42f3515..8084ea8 100644
--- a/classes/LocaleMiddleware.php
+++ b/classes/LocaleMiddleware.php
@@ -40,7 +40,7 @@ class LocaleMiddleware
                 && $parsedLocale[1]['language'] == $proposedLocale['language']
                 && $parsedLocale[1]['region'] == $proposedLocale['region']
             ) {
-                return $proposedLocale['language'].'_'.$proposedLocale['region'];
+                return new Locale($proposedLocale['language'].'_'.$proposedLocale['region']);
             }
         }
     }
@@ -61,10 +61,10 @@ class LocaleMiddleware
         if (!isset($curLocale)) {
             if (isset($headers[0])) {
                 $this->locale->setLocale(
-                    AcceptLanguage::detect([$this, 'testLocale'], 'en_US', $headers[0])
+                    AcceptLanguage::detect([$this, 'testLocale'], new Locale('en_US'), $headers[0])
                 );
             } else {
-                $this->locale->setLocale('en_US');
+                $this->locale->setLocale(new Locale('en_US'));
             }
         }
 
diff --git a/tests/LocaleMiddlewareTest.php b/tests/LocaleMiddlewareTest.php
index de54aee..275ff39 100644
--- a/tests/LocaleMiddlewareTest.php
+++ b/tests/LocaleMiddlewareTest.php
@@ -5,7 +5,9 @@
 
 namespace Alltube\Test;
 
+use Alltube\LocaleManager;
 use Alltube\LocaleMiddleware;
+use Slim\Container;
 use Slim\Http\Environment;
 use Slim\Http\Request;
 use Slim\Http\Response;
@@ -28,7 +30,9 @@ class LocaleMiddlewareTest extends \PHPUnit_Framework_TestCase
     protected function setUp()
     {
         $this->locale = getenv('LANG');
-        $this->middleware = new LocaleMiddleware();
+        $container = new Container();
+        $container['locale'] = new LocaleManager();
+        $this->middleware = new LocaleMiddleware($container);
     }
 
     /**
@@ -88,4 +92,22 @@ class LocaleMiddlewareTest extends \PHPUnit_Framework_TestCase
         $this->assertEquals('fr_FR', getenv('LANG'));
         $this->assertEquals('fr_FR', setlocale(LC_ALL, null));
     }
+
+    /**
+     * Test the __invoke() function withot the Accept-Language header.
+     *
+     * @return void
+     */
+    public function testInvokeWithoutHeader()
+    {
+        $request = Request::createFromEnvironment(Environment::mock());
+        $this->middleware->__invoke(
+            $request->withoutHeader('Accept-Language'),
+            new Response(),
+            function () {
+            }
+        );
+        $this->assertEquals('en_US', getenv('LANG'));
+        $this->assertEquals('en_US', setlocale(LC_ALL, null));
+    }
 }