Since upgrading my website to version 1.5.13, I can no longer upload images in my media manager.

I'm getting the following error message:

Fatal error: Call to a member function authorize() on a non-object in /home/*****/public_html/administrator/components/com_media/views/media/tmpl/default.php on line 64

I made no other changes to the site other than the upgrade...

Any ideas what might be causing this error and how to fix it?