Information cannot have keys other than those declared on the root type, but it doesn't

Question

Model builder is throwing the following error:

An exception of type 'System.InvalidOperationException' occurred in EntityFramework.Core.dll but was not handled in user code

Additional information: The derived type 'Namespace.myclass' cannot have keys other than those declared on the root type.

Context

 protected override void OnModelCreating(ModelBuilder modelBuilder)
 {

     modelBuilder.Entity<MyClass>((item) =>
     {
            item.HasKey(p => p.MyKey);
     });
 }

MyClass

public class MyClass
{
    public MyClass()
    {
        OtherClass = new List<OtherClass>();
    }

    public int MyKey { get; set; }
    public string Name { get; set; }

    public virtual ICollection<OtherClass> OtherClass { get; set; }
}