Entity Framework: field of composite key cannot be nullable?

Question

I have a model with composite key - the row is the key:

public class Item
{
    [Key, Column(Order = 0)]
    public int UserId { get; set; }
    [Key, Column         


        

Answer 1

Answer from Raphael lead me to another search. Here is the why it is not possible (answer from Cobsy):

What's wrong with nullable columns in composite primary keys?

In short: NULL == NULL -> false

Wierd. The solution for me is to add Id column into Model.

BTW: MySQL allow me not to define Primary Key, then I'm allowed to have such schema - EF complains about not defining the key :-(.