Component & Reveal

I have the following class:

public class BusinessTransaction
{
   private Hold _onHold;
}

And the following BusinessTransaction mapping:

Component(Reveal.Member("_onHold"))
   .ColumnPrefix("{property}_");

I also have a ComponentMap for Hold. When I build my session factory I get the following error: "Unable to find external component for 'Object', referenced from property '_onHold' of 'BusinessTransaction'."

Any thoughts?

Thanks,

Shane