Problem on Calcsums using CFRONT

Dear Experts,

I am using the cfront to retrived data from a table. I want to get the total sum of a field in all the record. But it always return error on the calcsums area. The Error is Calcsums Not implemented

This is a part of my code:

ocxCF.OpenTable(Table, 63001);
Record := ocxCF.AllocateRec(Table);

IF ocxCF.SetCurrentKey(Table, 'Customer No.,Type of Extension,Clear') THEN BEGIN
   IF ocxCF.FindRec(Table, Record, '-') THEN BEGIN
      IF ocxCF.CalcSums(Table, Record, 'ExtAmount') THEN
         MESSAGE(ocxCF.GetFieldData(Table, Record, 5))
      ELSE
         MESSAGE(ocx.GetErrText);
   END;
END ELSE
   ERROR(ocx.GetErrText);

in this code I have set the key I will be using, in this key it has sumindexfield = ExtAmount

Please tell me how/what is the correct format.

Thanks in Advanced