How do I use null in a struct?

[Václav Kozák via Digitalmars-d-learn]

I'm making a Rest API with vibe.d and I have a struct User. 
Sometimes I need to return only a few of the fields. So for 
example: return User(1, null, "John", null, null, ...);
If I do this, an error occurs: cannot implicitly convert 
expression null of type typeof(null) to ...
Thanks.