form not recognizing variable defined in query

Hi there -

My form does not seem to be recognizing a variable that I defined in a query. I'm getting a #NAME? error in the text field on the form I've created. It is not recognizing the [Minutes] variable that is created in a query. In testing I see that the values are calculating properly in the query, and that the formula in the form's text field is working if I plug in numeric values instead of the field names.

This is the variable expression entered in the Field row of the query design grid:
Minutes: DateDiff("n", [StartDateTime], [EndDateTime])

To display this value as hours and minutes, I'm using a text box with this Control Source:
=[Minutes] \ 60 & Format([Minutes] Mod 60, "\:00")

Any help would be much appreciated, thanks!

- Linton