[Zope] stressed wovels

I have this problem. I made a SQL method for storing strings into an Interbase field. If I insert a string containing a stressed wovels (like " è ") directly into the DB it works, but if I use my SQL method it gives me this error: Error, Products.gvibDA.gvib.gvibExceptions.DataError: - arithmetic exception, numeric overflow, or string truncation -- Cannot transliterate character between character sets What is the matter? Is it a GV Interbase adapter's bug?