Rank function based on OLAP universe

Hi,

I have an universe based on a BEx query. I want to add a rank measure.

My measure expression is:

{<EXPRESSION>Rank([0MATERIAL].[LEVEL01], Order([0MATERIAL].[LEVEL01], [Measures].[744AADNOFADBE3JP736AHGJU3]), BDESC))</EXPRESSION>}

First a warning comes up as:

The XML for this expression is correct, however the MDX expression is not parsed in this process. (UNV 00050).

When I use it in Web Intelligence, the error is:

Database error: The supplied XML is not valid. {<EXPRESSION>Rank([0MATERIAL].[LEVEL01], Order([0MATERIAL].[LEVEL01], [Measures].[744AADNOFADBE3JP736AHGJU3]), BDESC))</EXPRESSION>}. (IES 10901) (WIS 10901)

Any suggestions?