Field calculated with a data structure rule shows strange text

I have a database string field and I want to calculate its value with a data structure rule. The result should be either a sum of several other fields or a string constant, resolved by a complex data rule. When I open the screen the field shows the text Origam.XmlContainer and it doesn’t change even when all the fields needed for the calculation are filled.

Seems that you selected a Complex Data Rule while also selecting a Target Field. This is unsupported and should actually throw an exception.