diff --git a/src/Invoker.php b/src/Invoker.php
index 0c3d97d..b6005ce 100644
--- a/src/Invoker.php
+++ b/src/Invoker.php
@@ -135,7 +135,7 @@ private function resolveCallableFromContainer($callable)
                 return $this->container->get($callable);
             } else {
                 throw new NotCallableException(sprintf(
-                    '%s is neither a callable nor a valid container entry',
+                    '"%s" is neither a callable nor a valid container entry',
                     $callable
                 ));
             }
diff --git a/tests/InvokerTest.php b/tests/InvokerTest.php
index 3915df4..eed781d 100644
--- a/tests/InvokerTest.php
+++ b/tests/InvokerTest.php
@@ -284,7 +284,18 @@ public function should_throw_if_calling_non_callable_without_container()
     /**
      * @test
      * @expectedException \Invoker\Exception\NotCallableException
-     * @expectedExceptionMessage foo is neither a callable nor a valid container entry
+     * @expectedExceptionMessage NULL is not a callable
+     */
+    public function should_throw_if_calling_non_callable_without_container_2()
+    {
+        $invoker = new Invoker();
+        $invoker->call(null);
+    }
+
+    /**
+     * @test
+     * @expectedException \Invoker\Exception\NotCallableException
+     * @expectedExceptionMessage "foo" is neither a callable nor a valid container entry
      */
     public function should_throw_if_calling_non_callable_with_container()
     {