Ole & Excel 97

Hello,

Can someone help me with this problem.
I use delphi 3 with OLE to manipulate my data.
All works fine behalve one thing :

I need to change my default worksheet (in the same WorkBook) at a such
moment.

I try this command (documented in the help file of excel) :
XL.WORKSHEETS(2).ACTIVATE; or XL.WORKSHEETS('NEW SHEET').ACTIVATE;

I receive this message :Project1.exe raised exeption class EOleSysError
with message 'MEMBER NOT FOUND'. Process Stopped ...

I've tried a lot of syntax but none are working.

If someone has an idea ?
Thanks