Issue with lazy loading and inheritance in NHibernate

I have the following scenario in a C# application using NHibernate:

public abstract class A
{
    public virtual int SomeProperty { get; protected set; }

    public abstract void DoSomethingWithSomeProperty(int value);
}

public class B : A
{
    public override void DoSomethingWithSomeProperty(int value)
    {
        SomeProperty = value + 1;
    }
}

public class C : A
{
    public override void DoSomethingWithSomeProperty(int value)
    {
        SomeProperty = value + 10;
    }
}

The issue arises because NHibernate complains that method "DoSomethingWithSomeProperty" is non-virtual; I suppose this is because SomeProperty is virtual itself.

I'm aware one way I could circumvent this is by disabling lazy loading for these tables. However, this is undesirable because of the way the application was designed.

I want to force whoever inherits from A to implement DoSomethingWithSomeProperty. The best way I found to work around this issue is by making both B and C implement an interface that declares DoSomethingWithSomeProperty. But that just feels really dirty.

What is the best way to deal with this?