Add a calculated member with a custom formula

I am trying to add a calculated member 'Sunday Differential' and it would replicate the 'Regular' hours only on a Sunday in the "Sunday Differential" row. But somehow it is populating for all the days as shown.

The calculated formula I put in is

if {@Day} = "Sun"

then

GridValueAt(GetRowPathIndexOf({@TRC}), CurrentColumnIndex, CurrentSummaryIndex)

else

0

Would appreciate any guidance here. Thanks!