This seems like it ought to work but doesn't. MM.NET 2.4, strongly typed entitiy everywhere. Any ideas?

public class OrderLineEntity: ABusinessEntity
{
  public double? Price{ get {...} set {...} }
  public double? Quantity { get {...} set {..} }

  public double? Total { get { return Price*Quantity;} }
}

// .. then on form...
           this.mmTextBox4.BindingSource = "OrderLine";
            this.mmTextBox4.BindingSourceMember = "Total";

Bound Textboxes crash in mmBindingStrategy.cs trying to access ds.Tables[tableName].Columns[fieldName]. Bound label crashes with mysterious "Cannot bind to the property or column .... on the DataSource.
Parameter name: dataMember"
Grid columns show blank (probably internally one of the above exceptions is being silently caught: I haven't investigated this one because it seemed least likely to work in the first place).