Cannot insert the value NULL into column from unrelated entity

Question

I'm getting a "Cannot insert null into column" error from an unrelated table when trying to add a new record for the "original" table.

I have the following two (relevant) entities:

public class Complex
{
    [Key]
    public Guid Id { get; set; }
    public string Name { get; set; }

    public Guid OwnerId { get; set; }
    [ForeignKey("OwnerId")]
    public Owner Owner { get; set; }

    public Guid AddressId { get; set; }
    [ForeignKey("AddressId")]
    public virtual Address Address { get; set; }

    public virtual ICollection<Unit> Units { get; set; }
    public virtual ICollection<StaffMember> StaffMembers { get; set; }

    public Complex()
    {
        this.Id = System.Guid.NewGuid();
        this.Units = new HashSet<Unit>();
        this.StaffMembers = new HashSet<StaffMember>();
    }

    public void AddUnit(Unit unit)
    {
        Units.Add(unit);
    }

    public void AddStaff(StaffMember staffMember)
    {
        StaffMembers.Add(staffMember);
    }
}

and

public class Owner
{
    [Key]
    public Guid Id { get; set; }
    public string Name { get; set; }
    public Guid ContactInfoId { get; set; }
    [ForeignKey("ContactInfoId")]
    public ContactInfo ContactInfo { get; set; }

    public ICollection<StaffMember> Employees { get; set; }
    public ICollection<Complex> Complexes { get; set; }

    public Owner()
    {
        this.Id = System.Guid.NewGuid();
        this.Employees = new HashSet<StaffMember>();
        this.Complexes = new HashSet<Complex>();
    }

    public void AddEmployee(StaffMember employee)
    {
        Employees.Add(employee);
    }

    public void AddComplex(Complex complex)
    {
        Complexes.Add(complex);
    }
}

I'm trying to add a new owner in the following code:

if (ModelState.IsValid)
        {
            Owner newOwner = new Owner();
            ContactInfo newContactInfo = new ContactInfo();
            Address newAddress = new Address();

            newAddress.Address1 = viewModel.ContactInfo.Address.Address1;
            newAddress.Address2 = viewModel.ContactInfo.Address.Address2;
            newAddress.City = viewModel.ContactInfo.Address.City;
            newAddress.State = viewModel.ContactInfo.Address.State;
            newAddress.Zip = viewModel.ContactInfo.Address.Zip;

            newContactInfo.Address = newAddress;
            newContactInfo.Email = viewModel.ContactInfo.Email;
            newContactInfo.Phone1 = viewModel.ContactInfo.Phone1;
            newContactInfo.Phone2 = viewModel.ContactInfo.Phone2;

            newOwner.Name = viewModel.Name;
            newOwner.ContactInfo = newContactInfo;

            using (REMSDAL dal = new REMSDAL())
            {
                dal.Owners.Add(newOwner);

                var result = await dal.SaveChangesAsync();

                if (result > 0)
                {
                    viewModel.ActionStatusMessageViewModel.StatusMessage = "Owner " + viewModel.Name + " added.";
                    viewModel.Name = "";

                    return View(viewModel);
                }
            }
        }

...but getting this error:

Exception Details: System.Data.SqlClient.SqlException: Cannot insert the value NULL into column 'OwnerId', table 'REMS.dbo.Complexes'; column does not allow nulls. UPDATE fails. The statement has been terminated.

How can I be getting an error regarding Complexes when I'm trying to add an Owner?