Fixed a bug when detecting if the user used a callback.
diff --git a/system/core/Router.php b/system/core/Router.php
index a5d01b1..d6788c3 100644
--- a/system/core/Router.php
+++ b/system/core/Router.php
@@ -374,7 +374,7 @@
             if (preg_match('#^'.$key.'$#', $uri, $matches))
             {
                 // Are we using a callback?
-                $callable = is_callable($val);
+                $callable = ! is_string($val) && is_callable($val);
 
                 // Are we using callbacks to process back-references?
                 if($callable){