Dedupe fuzzy rule with Custom Fields

I need a dedupe rule that check Surname, Name and a Custom field i create, when a new contact is created by user interface.
If i use the default rule (Surname, Name and Email) everything goes fine but when instead of Email i use the custom field, even if i put the same data, Civicrm creates contacts without any warning or errors.
If i check manually by "Use rule" link in Find and Merge page, Civicrm find duplicate contacts.
What's up? I mistake something?
Tnx a lot.