R – SSRS – rsMultiReportItemsInPageSectionExpression Error

The Particulars:

I have a report that displays information about invoices. There is a page break between multiple invoices (each invoice gets its own page). What I want in the report header is the result of this expression (ex. "June, 2009"):

=MonthName(Month(ReportItems!textbox1.Value)) & ", " & cstr(Year(ReportItems!textbox1.Value))

But I get this exception (textbox2 is in the header):

Error 1 [rsMultiReportItemsInPageSectionExpression]
The Value expression for the textbox
‘textbox2’ refers to more than one
report item. An expression in a page
header or footer can refer to only one
report item.

First of all… Why would that even matter?! Second of all… How can I work around this strange restriction?

The Big Picture:

The reason I'm grabbing the text from another textbox is to work around the restriction that you can't use data fields in a header or footer. So there's a hidden column that shows the invoice date next to every transaction. Then the table header has the expression:

=First(Fields!InvoiceDate.Value, "table1_Group1")

This seems ridiculous and I hope I'm just doing something wrong. Yes I realize I could simply have a "month" and "year" text box in the header but that pushes the complexity way too high for the simple requirement of showing an invoice date in the header of a report.