fixed filename not being added to syntax error messages

CHANGELOG

@@ -15,6 +15,7 @@ Backward incompatibilities:
 
 Changes:
 
+ * fixed filename not being added to syntax error messages
  * added the autoescape option to enable/disable autoescaping
  * removed the newline after a comment (mimicks PHP behavior)
  * added a syntax error exception when parent block is used on a template that does not extend another one

lib/Twig/Error.php

@@ -21,7 +21,7 @@ class Twig_Error extends Exception
     protected $filename;
     protected $rawMessage;
 
-    public function __construct($message, $lineno = -1, $filename = 'n/a')
+    public function __construct($message, $lineno = -1, $filename = null)
     {
         $this->lineno = $lineno;
         $this->filename = $filename;
@@ -48,7 +48,7 @@ class Twig_Error extends Exception
     {
         $this->message = $this->rawMessage;
 
-        if ('n/a' != $this->filename) {
+        if (null !== $this->filename) {
             $this->message .= sprintf(' in %s', $this->filename);
         }