Rollup not working with lastPeriods function?

[RubenvdLinden]

I have a report on a PowerCube that contains a semi-additive measure 'Number of employees'.
The developer of the cube has calculated this measure using a rollup of the employees divided by the number of months in the selection.

My report uses the measure to display the average number of employees over the current year until a specified month. To filter on the year until the specified month, I have made a filter on the current year and a dataitem in which I use periodsToDate([Staff].[Period].[Period].[Month]; [Selected month]). This works fine.

Now, HRM have asked me to create a variant of this report in which the average number of employees is displayed over the last 12 months until the specified month. I removed the filter on the current year and changed my periodsToDate function to lastsPeriods(12; [Selected month]). The report runs, but the measure 'seems to forget' to divide itself by 12.

I can solve this by dividing my dataitem manually by 12, but it doesn't seem right.

Any help would be very appreciated.

[RubenvdLinden]

We upgraded to Cognos 8.4 and now it works.