Pls. help ... adding two calculated fields from two subforms within same main form

My problem looks simple but I can't get it done properly.
I have a main form with two subforms, Internal sales & External sales. Both forms have total boxes in the footer, Total internal sales (TIS) & Total external sales (TES). I added a new text box in the footer of second form (External sales) to calculate Total sales (TS), source expression: = [Forms]![Internal sales]!TIS + TES
It does not work in the main form, shows #Name! error. Only works when I have both subforms opened independently.

Solutions found on net but didn't work are:
1. Applying Me.Refresh or ReCalt expression on OnCurrent property of the external sales subform
2. Adding reference to main form in the expression, =[Forms]![Main form]![Internal sales].[Internal sales]!TIS + TES

Help please .....