Adobe LiveCycle Access Field in Subform inside Master Page

Hello,

I have a Dynamic PDF document created in LiveCycle 8.2 with the following structure

topmostSubform

          (Master Pages)

                  Page1

                        sfHeader

                               txtName

When I try to assign the value, using C# language, to the txtName I'm getting a null exception.

It looks that my code can't recognize the hierarchy structure.

I tried the following options and none of them worked

topmostSubform[0].#pageSet[0].Page1[0].sfHeader[0].txtName[0]"

topmostSubform[0].pageSet[0].Page1[0].sfHeader[0].txtName[0]"

topmostSubform[0].Page1[0].sfHeader[0].txtName[0]"

Any advise will be truly appreciated!

Thank you.