WriteBack Weighted Allocation doesn't work properly

Hello everyone,

I have an issue about weighted allocation. I'm trying to allocate on leafs of a user defined hierarchy which has the dimension key on the leaf level. The allocation have to be done based on the rolling 12 months. Below is the script I'm using on SSAS 2012:

UPDATE [CUBE] 
SET		( [Time].[YM].[Month Name].&[201312]	-- leaf
		, [Dim1].[Hierarchy1].[Level3].&[101]	-- to be allocated on the leafs ([Dim1].[Hierarchy1].[Level4])
		, [Measures].[Sales TG]			-- measure
		) = 10000 
USE_WEIGHTED_ALLOCATION BY -- get share of each leaf from level4 based on the total level3
	SUM((PARALLELPERIOD([Time].[YM].[Month Name], 11, [Time].[YM].[Month Name].&[201312]):[Time].[YM].[Month Name].&[201312]
		, [Dim1].[Hierarchy1].CURRENTMEMBER)
		, [Measures].[Sales]
		)       
		/       
	SUM((PARALLELPERIOD([Time].[YM].[Month Name], 11, [Time].[YM].[Month Name].&[201312]):[Time].[YM].[Month Name].&[201312]
		, [Dim1].[Hierarchy1].[Level3].&[101])
        , [Measures].[Sales]
        )

I saw some examples which don't use SUM, but if I remove the SUM operator I'm getting this error: 

"The  function expects a string or numeric expression for the  argument. A tuple set expression was used."

Using SUM will update only one cell in the level4, which is not what I am expecting from this approach.

Can you please help me with it?