ComponentAsId does not find property that belongs to a parent class

e.g.
public class Key
{
public string GroupId { get; set; }
public string NameId { get; set; }

public override bool Equals( object obj )
{
Key key = obj as Key;
if ( key == null )
{
return false;
}

return ( key.GroupId == GroupId && key.NameId == NameId );
}

public override int GetHashCode()
{
return GroupId.GetHashCode() + NameId.GetHashCode();
}
}

public abstract class NotMappedParent
{
public virtual Key Id { get; set; }
}

public class Child : NotMappedParent
{
public virtual string Value { get; set; }
}

mapper.Class<Child>(ca =>
{
ca.ComponentAsId(s => s.Id, map =>
{
map.Property(key => key.GroupId);
map.Property(key => key.NameId);
});
});

Actual :
<id type="Int32" />

Expected :
<composite-id class="Key" name="Id">
<key-property name="GroupId" />
<key-property name="NameId" />
</composite-id>