I have a form called "Bed-Tillage" with a lookup field called Section_Setup

This form has a subform called "Bed_Selection" with a lookup field called Section

What I want to happen: 

as records are added to the subform  "Bed_Selection"  I wold like the field Section to be auto filled by the parent form field  Section_Setup

I tried this in the on load of the subform

input.Section = input.Bed_Tillage.Section_Setup;

but it creates this error - 

Error at line number : 1 

'Bed_Tillage' is a lookup field and child fields cannot be accessed

Any help with this is appreciated.

Chris