MVC4: ModelState not valid when passing null for Nullable field
The problem came from the return value "null" instead of "" (empty string).I am using jtable (jtable.org) and created a custom display field for ParentID, which caused the problem, as I showed the ParentIDs (1,2,....44..) and "null" on top Level.So "null" got posted back, where it should have been ""