The tablix has a detail member with inner members. Detail members can only contain static members

I'm trying to create a simple document type report: The header is showing som info which should change based on the lines. I have a table (tablix) that is showing the lines in the body. I have created the Get/Set data functionality and it works...partly. I'm getting the same info on all the headers:-( By looking on som of the standard document reports, I have figured out that my hidden text-box and my table has to be inside a List (tablix), but when I try to move the (simple) tablix into the list I get this error when compiling the report:
The tablix has a detail member with inner members. Detail members can only contain static members

What am I missing here?