Booleans can't be empty. But in XML mappings this is really needed: XSD's often specify optional booleans. A workaround is to create an enumeration (true, false) and map that from/to a string, but then the XSD must also be manually changed so that booleans are replaced by strings. This is labour-intensive and not intuitive. Mendix should have support for mapping optional booleans.
This is still actual in current versions. I was surprised there were zero comments 2 years ago, so I was wondering if many other people are having to deal with this problem and what there solution is.
I have had this problem. The "solution" was not to import the boolean at all. Fortunately I don't need it yet, but will be a real pain if I do in future.