Have worked on this MDX Query for a while but is stuck! Can not figure out why SUM() on GENERATE will not work, error is:
"SUM function expects a tulpe set expression for argument 1, a string or numeric expression was used"

WITH
MEMBER [Measures].[ParameterCaption] AS [Invoice].[Faktura].CURRENTMEMBER.MEMBER_CAPTION
MEMBER [Measures].[ParameterValue] AS [Invoice].[Faktura].CURRENTMEMBER.UNIQUENAME
MEMBER [Measures].[ParameterLevel] AS [Invoice].[Faktura].CURRENTMEMBER.LEVEL.ORDINAL
MEMBER [Measures].[Fakturabelopp1] AS SUM(
IIF(
[Invoice].[Faktura].CURRENTMEMBER.LEVEL.ORDINAL = 2, ([Measures].[HeadAmount], [Invoice].[Faktura].FIRSTCHILD),
IIF(
[Invoice].[Faktura].CURRENTMEMBER.LEVEL.ORDINAL = 1,
GENERATE(
DESCENDANTS([Invoice].[Faktura].CURRENTMEMBER, [Invoice].[Faktura].[Fakturanummer]),
(DESCENDANTS([Invoice].[Faktura].CURRENTMEMBER, [Invoice].[Faktura].[Fakturarad]).ITEM(0), [Measures].[HeadAmount])
)
, NULL)
))
SELECT {
[Measures].[ParameterCaption],
[Measures].[ParameterValue],
[Measures].[ParameterLevel],
[Measures].[Fakturabelopp1]
} ON COLUMNS,
(
[Bolag].[Bolag].[Bolag].&[G10]*[Invoice].[Faktura].[Fakturaserie]
)
DIMENSION PROPERTIES MEMBER_CAPTION, MEMBER_UNIQUE_NAME ON ROWS
FROM [Visma Control]
WHERE ([Tid].[Tid].[År].&[4])

Result with sum:
G10 - AB G10KR G10KR [Invoice].[Faktura].[Fakturaserie].&[G10KR] 1 #Error
G10 - AB G10KRI G10KRI [Invoice].[Faktura].[Fakturaserie].&[G10KRI] 1 #Error

Result without sum:
G10 - AB G10KR G10KR [Invoice].[Faktura].[Fakturaserie].&[G10KR] 1 92775,8669501526,73734,423515,831256,119056
G10 - AB G10KRI G10KRI [Invoice].[Faktura].[Fakturaserie].&[G10KRI] 1 12338538,4320612,54200038000420002-199511937,51
G10 - AB G12KR G12KR [Invoice].[Faktura].[Fakturaserie].&[G12KR] 1 (null)
G10 - AB G12KRI G12KRI [Invoice].[Faktura].[Fakturaserie].&[G12KRI] 1 (null)