diff --git a/tests/Forward/Builder.php b/tests/Forward/Builder.php
index f6956fc..b6a4874 100644
--- a/tests/Forward/Builder.php
+++ b/tests/Forward/Builder.php
@@ -19,6 +19,11 @@ public function make()
 
         $app->get('/', 'Rougin\Slytherin\Fixture\Classes\NewClass@index');
 
+        $app->get('/hello', function ()
+        {
+            return 'Hello world!';
+        });
+
         return $app;
     }
 
diff --git a/tests/Forward/ForwardTest.php b/tests/Forward/ForwardTest.php
index 003db7b..3aae073 100644
--- a/tests/Forward/ForwardTest.php
+++ b/tests/Forward/ForwardTest.php
@@ -36,4 +36,18 @@ public function test_default_route()
 
         $this->builder->make()->run();
     }
+
+    /**
+     * @runInSeparateProcess
+     *
+     * @return void
+     */
+    public function test_route_as_callback()
+    {
+        $this->builder->setUrl('GET', '/hello');
+
+        $this->expectOutputString('Hello world!');
+
+        $this->builder->make()->run();
+    }
 }
diff --git a/tests/Http/MessageTest.php b/tests/Http/MessageTest.php
index 18199c5..1d1a109 100644
--- a/tests/Http/MessageTest.php
+++ b/tests/Http/MessageTest.php
@@ -88,10 +88,6 @@ public function test_getting_protocol_version()
      */
     public function test_adding_a_header()
     {
-        $message = $this->message->withAddedHeader('age', '18');
-
-        $this->assertTrue($message->hasHeader('age'));
-
         $expected = array('Rougin', 'Royce', 'Gutib');
 
         $message = $this->message->withAddedHeader('names', $expected);
@@ -101,6 +97,16 @@ public function test_adding_a_header()
         $this->assertEquals($expected, $actual);
     }
 
+    /**
+     * @return void
+     */
+    public function test_checking_a_header()
+    {
+        $message = $this->message->withAddedHeader('age', '18');
+
+        $this->assertTrue($message->hasHeader('age'));
+    }
+
     /**
      * @return void
      */
diff --git a/tests/Middleware/DispatcherTestCases.php b/tests/Middleware/DispatcherTestCases.php
index 8cd9138..f471b37 100644
--- a/tests/Middleware/DispatcherTestCases.php
+++ b/tests/Middleware/DispatcherTestCases.php
@@ -4,6 +4,8 @@
 
 use Rougin\Slytherin\Http\ServerRequest;
 use Rougin\Slytherin\Middleware\Interop;
+use Rougin\Slytherin\Middleware\Multiple\Interop05;
+use Rougin\Slytherin\Middleware\Multiple\Slytherin;
 use Rougin\Slytherin\Middleware\Wrapper;
 use Rougin\Slytherin\System\Lastone;
 use Rougin\Slytherin\Testcase;
@@ -179,6 +181,22 @@ public function test_getting_middlewares()
         $this->assertCount(1, $actual);
     }
 
+    /**
+     * @return void
+     */
+    public function test_different_middleware_interfaces()
+    {
+        $expected = array('Rougin Gutib');
+
+        $items = array(new Slytherin, new Interop05);
+
+        $this->dispatcher->push($items);
+
+        $actual = $this->process()->getHeader('name');
+
+        $this->assertEquals($expected, $actual);
+    }
+
     /**
      * Processes the defined middleware dispatcher and return its response.
      *
diff --git a/tests/Middleware/Multiple/Interop05.php b/tests/Middleware/Multiple/Interop05.php
new file mode 100644
index 0000000..9a9bff7
--- /dev/null
+++ b/tests/Middleware/Multiple/Interop05.php
@@ -0,0 +1,34 @@
+<?php
+
+namespace Rougin\Slytherin\Middleware\Multiple;
+
+use Interop\Http\ServerMiddleware\DelegateInterface;
+use Interop\Http\ServerMiddleware\MiddlewareInterface;
+use Psr\Http\Message\ServerRequestInterface;
+
+/**
+ * @package Slytherin
+ * @author  Rougin Gutib <rougingutib@gmail.com>
+ */
+class Interop05 implements MiddlewareInterface
+{
+    /**
+     * @param  \Psr\Http\Message\ServerRequestInterface         $request
+     * @param  \Interop\Http\ServerMiddleware\DelegateInterface $delegate
+     * @return \Psr\Http\Message\ResponseInterface
+     */
+    public function process(ServerRequestInterface $request, DelegateInterface $delegate)
+    {
+        /** @var array<string, string> */
+        $parsed = $request->getParsedBody();
+
+        $response = $delegate->process($request);
+
+        if (array_key_exists('name', $parsed))
+        {
+            $response = $response->withHeader('name', $parsed['name']);
+        }
+
+        return $response;
+    }
+}
diff --git a/tests/Middleware/Multiple/Slytherin.php b/tests/Middleware/Multiple/Slytherin.php
new file mode 100644
index 0000000..0c27641
--- /dev/null
+++ b/tests/Middleware/Multiple/Slytherin.php
@@ -0,0 +1,31 @@
+<?php
+
+namespace Rougin\Slytherin\Middleware\Multiple;
+
+use Psr\Http\Message\ServerRequestInterface;
+use Rougin\Slytherin\Middleware\HandlerInterface;
+use Rougin\Slytherin\Middleware\MiddlewareInterface;
+
+/**
+ * @package Slytherin
+ * @author  Rougin Gutib <rougingutib@gmail.com>
+ */
+class Slytherin implements MiddlewareInterface
+{
+    /**
+     * @param  \Psr\Http\Message\ServerRequestInterface      $request
+     * @param  \Rougin\Slytherin\Middleware\HandlerInterface $handler
+     * @return \Psr\Http\Message\ResponseInterface
+     */
+    public function process(ServerRequestInterface $request, HandlerInterface $handler)
+    {
+        /** @var array<string, string> */
+        $parsed = $request->getParsedBody();
+
+        $parsed['name'] = 'Rougin Gutib';
+
+        $request = $request->withParsedBody($parsed);
+
+        return $handler->handle($request);
+    }
+}