Still can't get this.

I have a main menu form (MainMenu) on which there are two controls, a
dropdown (SelectedID) and a button.

The dropdown (SelectedID) does not have a control source because it is just
a list to select from.  The row source is a table (Customers) that has only
two fields, CustNo (text) and CustName (text).  The control is bound by the
first column, CustNo.

The button runs a macro that opens another form (CustDetail) and uses
OpenForm with a Where criteria of
[CustomerID]=[Forms]![MainMenu]![SelectedID].

The detail form (CustDetail) is bound to a table (Budget) where there are
two fields, CustomerID (text), and BudgetAmt (currency).

When I click the button, I get a dialog box asking me to enter the value for
[Forms]![MainMenu]![SelectedID].

I just don't get it!!!!  What am I doing wrong?!  Is this because the
dropdown on the main menu form doesn't have a control source?!

Help!