drupal_get_normal_path() not working

I'm having a problem with the pingback module and I've traced it down to drupal_get_normal_path() not being able undo to some path aliases. When this function gets something like ca/blogs/ivan/2009/09/26/hui-estrene-nou-lloc-web, it ends up calling custom_url_rewrite_inbound, which in turn calls subpath_alias_url_inbound_alter() and url_alter_url_inbound_alter(). The problem is that the first one isn't able to map its argument to node/NNN.

However, when the argument doesn't include the language component (e.g. blogs/ivan/2009/09/26/hui-estrene-nou-lloc-web), the path is correctly mapped to node/NNN.

Sorry if this is not the fault of the Sub-path aliases module, I'm a bit at a loss here. :( In this case, could you please point me to a more likely module causing this erroneous behaviour? Thanks.