Unique field for URLs: verifying fields composed by arrays

Hi!

First of all, congratulations for the module, it is incredibly useful!

I am trying to use it to verify that an URL field is also unique (with respect to the rest of the nodes of the same content type). However, I am getting the following error:

Warning: strlen() expects parameter 1 to be string, array given in unique_field_match_value() (line 281 of /home/drozas/workspace/cbpp-directory-code/sites/all/modules/contrib/unique_field/unique_field.module).

I am assuming this is due to the field being composed of arrays, rather than a single string (with the title works pretty well). Does anyone have any suggestion on how this module could be used to verify that a field of type URL is unique? In the worst case, I was thinking of creating a custom module, but I thought I might be missing a possible way to do it via the configuration with this module.

Thanks a lot!