Directory tag link causes error

I've created a directory and tagged each listing with State, City and ZIP.

When I view a listing and click through the tags for State & City it works fine, however if I click the ZIP code I get the following error:

Catchable fatal error: Object of class WP_Error could not be converted to string in /home/content/67/10196467/html/wp-content/plugins/directory/core/template-tags.php on line 157

Here is a sample page:

http://loweralabama.biz/listing/la-cosina-mexican-restaurant-mobile-al/

Many thanks
Ian.