Expression.Error: We cannot apply field access to the type Date.

Hi, I'm trying to change values in a column, based from another column. In more detail, I have a column "Date", and another column "Index". I want to add "Index" months to the "Date".

I know I can create a new column, based from Date+Index, and then delete the first two, but it's a waste of processing.

I have already this formula:

= Table.TransformColumns(#"ChangeDate",{{"DataDoc", each (Date.AddMonths([DataDoc],each [Index])) }})

but it gives me this error:

Expression.Error: We cannot apply field access to the type Date.

Help...