Warning: array_key_exists

hello, i installed viajes-verde-16 theme and when i write new post it show something like that:

“Warning: array_key_exists() [function.array-key-exists]: The first argument should be either a string or an integer in /home.3/u/s/i/usiuznte/www/blog/wp-includes/category-template.php on line 176”

this is screen: http://img110.imageshack.us/img110/232/whatnc3.jpg

how can i fix it??

cheers