tried to get a value from a SapGuiGrid object. I tried to use Output value, but QTP said that I can not edit Output Value. Invalid Pointer

So I tried to use the next function

VrBillDoc = SAPGuiSession("Session").SAPGuiWindow("Maintain Billing Due List").SAPGuiGrid("GridViewCtrl").GetCellData(1,"S ales Document")

QTP said that it can not find the object. However if I use ColumnCount event, Qtp can recognize my sapguiGrid.

I think that I have column name wrong. I need to know how I can get the column name for SAPGuiGrid. Or How can i get data from SAPGuiGrid.

p.d. I already try to use #index instead of Column name but I get the same message.

Thank you in advance, any comments will be greatly appreciated