I'm getting the following error

Notice: is_tax was called incorrectly. Conditional query tags do not work before the query is run. Before then, they always return false.

Please see Debugging in WordPress for more information. (This message was added in version 3.1.0.) in /home/username/public_html/wp-includes/functions.php on line 4161

The weird part is that is_tax, is not mentioned once in my functions.php file:

Nor is if found in the required option.php file:

What is causing this error and how can I alleviate it?