"The argument type 'Users? Function(User)' can't be assigned to the parameter type 'Users Function(User?)'."

Question

If I make the return type nullable, then when I .map(_userFromFirebaseUser) I'm getting an error that says "The argument type 'Users? Function(User)' can't be assigned to the parameter type 'Users Function(User?)'."

Users? _userFromFirebaseUser(User user){
    if (user != null){
       return Users(uid: user.uid);
    }
    return null;
}

Stream<Users?> get user {
    return _auth.authStateChanges().map(_userFromFirebaseUser);
}