Sub Report changes Name of Group Header

Thanks for taking the time to read my question.

I have a report with 2 fields in it. They sit in the Header of the field
MORD (a number). I used Sorting and Grouping to make the header. I

n the detail I want to put the subreport which is linked to the main report
by linking MORD to OrderNumber (which is in the sub report).

When I add the sub report and either let Access define my link, or define my
own link, then try to preview the report, it asks me for OrderNumber in a
dialog box. There is no reason for this. If I don't have the sub report in
the report, it doesn't ask me. If I open the subreport on it's own, I don't
get this prompt either.

Then I noticed that the header name of my group on MORD is now OrderNumber.
Access changes it. I don't know why. So the prompt is a result of Access
not being able to find the field "OrderNumber" in the main report, which it
shouldn't be able to, as it is not there.

Why is Access2000 doing this? Is my procedure in adding the subreport wrong?
I'm using the SubReport/SubForm button in the ToolBox to add the subreport.

Thanks again for taking the time to read this long post.

Brad