GV Version 9.1A (130402)

I am creating a View from the Sales Analysis data.

The following formula works properly (as I would expect):
INCLUDEAMOUNT( {Amt}, {Date}, "Y", "F" )

However, when I copy and modify the formula as displayed, to pick up values for the previous year, I receive an “Evaluation Failed” error message.
INCLUDEAMOUNT( {Amt}, {Date}, "LY", "F" )

When the formula is written with a “C” (Calendar Year) instead of “F” (Fiscal Year), no errors.

What’s going on?