Using a total() function and a multiselect prompt to roll up values

[beerguy82]

All -

I received a report request where the user wants to be be able to be prompted for discreet time periods, select multiple, and have a measure roll-up across those time periods.  I have tried doing it as follows, but it doesn't seem to work.  Any other ideas?

total([Quantity] for [Package Type],(#promptmany('pTimePeriods')#))

Thanks.

Tim