Can't select value from valuelist

Hi,

I have a valuelist based on a relation.

relation: table A category → Table B category
valuelist: relation Show in field A
return in dataprovider

In my form I have a combobox with that valuelist attached. When I select the field, the valuelist is displaying correctly, but when I select a value It leaves my field blank.
If I base the valuelist from the same table without relation it works without problems.

Am I missing something? :?: