plugin causes call_user_func_array error

The plugin to determine visitor location via IP was demanding a lot from my DB, so it seemed like the obvious solution was to wrap it in a script which first checked if the location was set in a cookie, and if not, invoke the visitor location plugin, and set it in a cookie. This plugin was super-short, and seemed within my abilities.

However, turning on my new plugin causes Warning: call_user_func_array() : First argumented is expected to be a valid callback, 'location_isset()' was given in /blah/blah/blah/wordpress/wp-includes/plugin.php</b> on line 311 Renaming it causes it to go away.

I’m hoping I’ve made a very stupid error which someone will easily be able to point out.