Error using summation in MDX query

Summary:

I'm trying to pull data from Essbase using a group of members that I need to sum up. To do it, I'm using WITH statement that looks something like that:

With 

Member [Risk] As '([Risk Type A] + [Risk Type C] + [Risk Type F])'

Then this [Risk] member will be used in a SET statement, together with other standard members of the hierarchy.

When I run this query through SmartView "Manage and Execute Queries" option, I'm getting this error message:

What am I doing wrong? I also tried to use SUM function but got the same result.

Thanks!