Environment: SQL-Server-2005, SSAS 2005

I am using the following formula to create a Moving Annual Total

`SUM ({[Time].CurrentMember.Lag(3):[Time].CurrentMember}, [Measures].[TRx Quantity])`

The same column `[Measures].[Trx Quantity]` is used in other calculations and it works fine. This one, however, comes up with `#Value!` to indicate some kind of an error. Can anyone point me in the proper direction here?