Different entities having an optional foreign key to the same table

Question

I have two tables that have a foreign key to another table, but both could be optional. I'm trying to do it through attributes.

Here are the two entities

public class Subscription : BaseModel
{
    public virtual User User { get; set; }
    public Guid UserId { get; set; }

    public DateTime ValidFrom { get; set; }
    public DateTime ValidUntil { get; set; }

    [ForeignKey("Regarding")]
    public virtual ICollection<RoleAssociation> Roles { get; set; } = new List<RoleAssociation>();
}


public class User : BaseModel
{
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Email { get; set; }

    [ForeignKey("Regarding")]
    public virtual ICollection<RoleAssociation> Roles { get; set; } = new List<RoleAssociation>();
}

And the table for RoleAssociation

public class RoleAssociation : BaseModel
{
    public Guid Regarding { get; set; }
           
    public virtual Application Application { get; set; }
    public Guid ApplicationId { get; set; }

    public virtual Functionality Functionality { get; set; }
    public Guid FunctionalityId { get; set; }

    public virtual Role Role { get; set; }
    public Guid RoleId { get; set; }

}

This works and creates the correct database schema, but when I'm trying to add a RoleAssociation and providing a Guid from a User for example, I'm being returned the following

The INSERT statement conflicted with the FOREIGN KEY constraint 
"FK_RoleAssociation_Subscription_Regarding". The conflict occurred in database 
"SecurityGateway", table "dbo.Subscription", column 'Id'.

It expects that I ALSO add a RoleAssociation for a Subscription which is not what I want.

I have thought of making Regarding optional, but I don't want any rows in that table to not have a value for this column, this would make no sense.

Any ideas ?