array_key_exists() on one theme

I just upgraded to to version 2.3 from 2.0.11 The upgraded fixed all of hte problems I had been having with various themes. Unfortunately I now have only one error left to correct on one theme. below is the error

Warning: array_key_exists(): The first argument should be either a string or an integer in /home/blah/public_html/WordPress/wp-includes/category-template.php on line 176

even though it says the error is in the category-template.php. Since the error only occurs on one template I’m assuming it has something to do with one of the pages in the theme. I’m new to using word press any help with how I can fix this would be much appreciated.