When SetIdMember is called with a memberMap argument that is for a different class the current error message is too cryptic.

This can be reproduced using these classes:

public class B
{
    public ObjectId Id;
    public int b;
}

public class C : B
{
    public int c;
}

and this code:

BsonClassMap.RegisterClassMap<C>(cm =>
{
    cm.AutoMap();
    cm.SetIdMember(cm.GetMemberMap(c => c.Id)); // wrong: Id is in class B, not class C
});

The call to SetIdMember is incorrect because it is being called when registering class C but Id is defined in class B. SetIdMember should have been called when registering class B instead.

The current error message "Invalid memberMap." is too cryptic. A better error message would provide more of an explanation of what is wrong.