FIX: The UPDATE CUBE MDX statement writes the remainder value of the writeback formula to a NULL cell

UPDATE CUBE
[Budget] SET  
( [Category].[All Category].[Forecast],  
[Account].[All Account].[Net Income].[Total Expense].[General & Administration],
[Store].[All Stores].[USA].[CA], 
[Time].[1998],
[Measures].[Amount]  ) = 750000

USE_WEIGHTED_ALLOCATION BY 
iif(
IsEmpty(( ParallelPeriod([Time].[Year], 1, [Time].Currentmember), 
[Category].[All Category].[Current Year's Actuals], [Store].Currentmember, 
[Account].[All Account].[Net Income].[Net Sales].[Gross Sales], [Measures].[Amount])),
-- Sales Value is empty for the previous year so leave the new value as NULL.
NULL, 
-- Otherwise take the Current Years Actual Gross Sales for the current store, 
--divide it by the total for all of California to get the percentage contribution 
--of this store to Gross Sales and allocate that percentage of the new 
--General & Administration value to this store.
( ParallelPeriod([Time].[Year], 1, [Time].Currentmember) ,  
[Category].[All Category].[Current Year's Actuals], [Store].Currentmember, 
[Account].[All Account].[Net Income].[Net Sales].[Gross Sales], [Measures].[Amount] )    /  
([Time].[1997], [Category].[All Category].[Current Year's Actuals], 
[Store].[All Stores].[USA].[CA], [Account].[All Account].[Net Income].[Net Sales].[Gross Sales], [Measures].[Amount] )  )

SELECT
[Measures].Members   ON COLUMNS,
{ [Store].[All Stores].[USA].[CA],   Descendants([Store].[All Stores].[USA].[CA], [Store].[Store City])  } 
ON ROWS
FROM [Budget]
WHERE 
( [Category].[All Category].[Forecast], 
[Time].[1998] , 
[Account].[All Account].[Net Income].[Total Expense].[General & Administration] )