Entity Framework 'column does not allow nulls'

Yet again the amazing Entity Framework has failed to make something simple, simple... I have created a really basic set of models, Company, County, User's have a M:1 relationship with Company. So I have an extra table I created called CompanyUsers that I believe will generate the table design I like.

However now I cannot enter a basic user into the database claiming that I cannot have a null UserId even though I am setting the Id.

I understand that my table design is weird, but I may eventually want to switch to a M:M relationship and having this table setup is nice for that. Secondly, having the data in 3 tables makes writing SQL queries easier because of joins.

Exact Error:

System.Data.SqlClient.SqlException: Cannot insert the value NULL into column 'UserId', table 'SSI.Database.Context.Software.dbo.Users'; column does not allow nulls. INSERT fails.

Seed:

var _company = new Company() 
{
    CompanyId=1,
    Name="Company Inc."
};

var _counties = new List<County>() {
    new County(){ Name="Place1" },
    new County(){ Name="Place2"}
};

context.Companies.AddOrUpdate(_company);

var _user = new User() {UserId=1, CompanyId=_company.CompanyId, Company=_company, Name = "John Smith" };


if (context.Counties.Count() == 0)
{
    context.Counties.AddRange(_counties);
}

context.Users.AddOrUpdate(_user);

All my models:

Company:

public class Company
{

    public Company()
    {
        Users = new List<User>();
    }

    [Key]
    public int CompanyId { get; set; }
    public string Name { get; set; }

    public ICollection<User> Users { get; set; }
}

User:

public class User
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int UserId { get; set; }
    public string Name { get; set; }


    public Nullable<int> CompanyId { get; set; }

    [ForeignKey("CompanyId")]
    public Company Company { get; set; }
}

CompanyUser Table:

public class CompanyUsers
{
    [Key]
    [Column(Order = 1)]
    public int CompanyId { get; set; }
    [ForeignKey("CompanyId")]
    public virtual Company Company { get; set; }
    [Key]
    [Column(Order = 2)]
    public int UserId { get; set; }
    [ForeignKey("UserId")]
    public virtual User User{ get; set; }
}