fixed a PHP notice when trying to access a key on a non-object/array variable

test/Twig/Tests/TemplateTest.php

@@ -10,6 +10,20 @@
  */
 class Twig_Tests_TemplateTest extends PHPUnit_Framework_TestCase
 {
+    /**
+     * @expectedException        Twig_Error_Runtime
+     * @expectedExceptionMessage Impossible to access a key ("a") on a "string" variable
+     */
+    public function testAttributeOnAString()
+    {
+        $template = new Twig_TemplateTest(
+            new Twig_Environment(null, array('strict_variables' => true)),
+            false
+        );
+
+        $template->getAttribute('string', 'a', array(), Twig_TemplateInterface::ARRAY_CALL, false);
+    }
+
     /**
      * @dataProvider getGetAttributeTests
      */