Passing data from Header to body

I am trying to pass data from a textbox in the header to a textbox in the body.

In the body I call a function using:

=Code.DoThis(ReportItems, "textbox3")

In Report...Report Properties...code I have this function

public shared function DoThis(byref reportItems as
Microsoft.ReportingServices.ReportProcessing.ReportObjectModel.ReportItems,
byref param1 as string) as string

Test = ReportItems(param1).value

Return Test

End Function

If I pass a textbox from the body, it works. If I pass the textbox from the
header,
I get #Error.

Any help would be greatly appreciated.