error: convert null to set type, tuple type

Hi everyone
I’m new to cognos and I have a problem in report studio

I wrote a mdx query for computing last nonempty value for a measure:
tuple(

ITEM(

tail(

FILTER(

[FACTACCINMONTH_94_Q3].[DIMDATE].[CALENDAR].[MONTH]
,
[Balanceinend]>0
)
,1),0)

,
[Balan...