Cannot Get Parameter to Display in Page Header

I have started a simple report that has two datasets. The first dataset just
returns all of the production orders. The second dataset is the main source
of the reports data and has production order as its parameter. For this
parameter, I have chosen the first dataset as the report parameter's source.
This works fine, and I am provided with a drop down box with all of the
production orders in the system. However, when I attempt to add a text box
to my existing Page Header that references the value of this report
parameter, the preview mode doesn't show my header at all. As soon as I
remove the text box, the header displays ok. The expression for the header
is =Parameters!ProdOrdNum.Value.
Am I missing something simple here? Thanks!Also, if I cut and paste the text box from the Header into the Body it
displays fine.