diff --git a/src/Container/Container.php b/src/Container/Container.php
index 9af5c1e6..59a17a04 100644
--- a/src/Container/Container.php
+++ b/src/Container/Container.php
@@ -38,7 +38,7 @@ class Container implements ContainerInterface
             return $this->resolve($id);
         }
 
-        throw new NotFoundException(sprintf('The entry with the id "%s" could not be found'));
+        throw new NotFoundException(sprintf('The entry with the id "%s" could not be found', $id));
     }
 
     /**
diff --git a/tests/Unit/Container/ContainerTest.php b/tests/Unit/Container/ContainerTest.php
new file mode 100644
index 00000000..f0ba24e7
--- /dev/null
+++ b/tests/Unit/Container/ContainerTest.php
@@ -0,0 +1,98 @@
+<?php
+
+namespace Engelsystem\Test\Config;
+
+use Engelsystem\Container\Container;
+use PHPUnit\Framework\TestCase;
+
+class ContainerTest extends TestCase
+{
+    /**
+     * @covers \Engelsystem\Container\Container::get
+     */
+    public function testGet()
+    {
+        $container = new Container();
+        $class = new class
+        {
+        };
+
+        $container->instance('foo', $class);
+        $this->assertSame($class, $container->get('foo'));
+    }
+
+    /**
+     * @covers \Engelsystem\Container\Container::get
+     * @expectedException \Engelsystem\Container\NotFoundException
+     */
+    public function testGetException()
+    {
+        $container = new Container();
+
+        $container->get('not.registered.service');
+    }
+
+    /**
+     * @covers \Engelsystem\Container\Container::instance
+     * @covers \Engelsystem\Container\Container::resolve
+     */
+    public function testInstance()
+    {
+        $container = new Container();
+        $class = new class
+        {
+        };
+
+        $container->instance('foo', $class);
+        $this->assertSame($class, $container->get('foo'));
+    }
+
+    /**
+     * @covers \Engelsystem\Container\Container::has
+     */
+    public function testHas()
+    {
+        $container = new Container();
+
+        $this->assertFalse($container->has('test'));
+
+        $class = new class
+        {
+        };
+
+        $container->instance('test', $class);
+        $this->assertTrue($container->has('test'));
+    }
+
+    /**
+     * @covers \Engelsystem\Container\Container::singleton
+     */
+    public function testSingleton()
+    {
+        $container = new Container();
+        $class = new class
+        {
+        };
+
+        $container->singleton('foo', $class);
+        $this->assertSame($class, $container->get('foo'));
+        $this->assertSame($class, $container->get('foo'));
+    }
+
+    /**
+     * @covers \Engelsystem\Container\Container::setInstance
+     * @covers \Engelsystem\Container\Container::getInstance
+     */
+    public function testContainerSingleton()
+    {
+        $container0 = new Container();
+        $container = Container::getInstance();
+
+        $this->assertNotSame($container0, $container);
+
+        $container1 = new Container;
+        Container::setInstance($container1);
+
+        $this->assertSame($container1, Container::getInstance());
+    }
+}