[Zope] getitem and string.lower syntax

Hi, Can somebody help me to correct what is wrong in this code below? <dtml-var expr="_.getitem(_.string.lower(date_code,1))"> date_code is a string value enter by the user by filling a form which i then reference as a DTML Method. This worked perfectly before inserting the string function -<dtml-var expr="_.getitem(date_code,1)"> then the user will have to type the value in a lower case, this i considered to be rigid. But after inserting the string function this is error msg i received Error Type: TypeError Error Value: read-only character buffer, tuple Thanks. Hamzat kamal Dnet Systems Ltd., http://www.dnetsystems.net.