Location Province getting cleared

I've written a little function to update some data on a custom content type, basically:

$thisnode = node_load($nid);

$thisnode->custom_type_field = $newvalue;

node_save($thisnode);

When the node gets saved, the location province value gets cleared. I'm not updating any of the location fields in this function so I was curious why the province field is effected and no other field is.