String to member

[Fast]

I'm running a report by a parameterized url.

When filtering a measure for a specific week everything works fine but for a ytd-measure calculated in the cube the report is empty. The ytd-calculation only seems to work when using a MUN. The problem is that the parameter only passes the member caption.

The filter looks like: roleValue('_memberCaption';[Week]) = ?paramWeek?

It's no problem to built a text string to look like a MUN but how can I get Cognos to treat the text as a MUN?

Regards,
Fast