Wordpress referring urls back to itself with /null on the end, resulting in 404.

Hi there, I've got a Wordpress install (3.3.2) and Shopp plugin (2.1) - I'm getting loads of url's, apparently referred from the site address that are resulting in 404 errors.

For example,
example.com

example.com/shop/null

example.com/shop/product/null

They all say the referrer is example.com

I've tried to circumvent this by setting up an .htaccess regex to get rid of null, which works but not from the site URL. Does anyone have any idea where this is coming from? or alternatively, how can i adapt my regex to redirect example.com/null back to example.com

I'm more worried about how this is happening though.... any ideas?

Thanks!