What's difference between this two calculated member?

Hi guys,

Anyone can help to explain why I cannot use [Date].[Calendar].[Month].CurrentMember  but [Date].[Calendar].CurrentMember

Because on rows there's {[Date].[Calendar].[Month].Members} , I should be the member under [Month] level, so I use CurrentMember under [Month] in calculated MEMBER to get the current month under [Date].[Calendar].[Month].

What's wrong with this definition?

WITH MEMBER [Measures].[Reseller Running Total Error]
AS
AGGREGATE(
	PERIODSTODATE([Date].[Calendar].[Calendar Year],
					[Date].[Calendar].[Month].CurrentMember 
					),
	([Measures].[Reseller Sales Amount])
	)
MEMBER [Measures].[Reseller Running Total]
AS
AGGREGATE(
	PERIODSTODATE([Date].[Calendar].[Calendar Year],
					[Date].[Calendar].CurrentMember 
					),
	([Measures].[Reseller Sales Amount])
	)
SELECT
{
([Measures].[Reseller Sales Amount]),
([Measures].[Reseller Running Total]),
([Measures].[Reseller Running Total Error])
} ON COLUMNS,
{[Date].[Calendar].[Month].Members} ON ROWS
FROM [Step-by-Step]

Results 

Error Information -

CellOrdinal 2
VALUE #Error Query (5, 13) The CURRENTMEMBER function expects a hierarchy expression for the 1 argument. A member expression was used.
FORMATTED_VALUE #Error Query (5, 13) The CURRENTMEMBER function expects a hierarchy expression for the 1 argument. A member expression was used.

Thanks!

Please vote if it's helpful and mark it as an answer!