SubForm Updated From ListBox

I have subform which uses a SQL statement as the record source.
One of the criteria (ie. Part of the WHERE condition) is a column value from a ListBox on the MainForm.

Two problems exist:
1. When the MainForm opens, I get a "Enter Parameter Value" window, asking me to input
the value it should be getting from the ListBox. (I suppose this is fair enough, as the ListBox hasn't been
created/populated yet!)
2. When the ListBox is there, when I click on a row, I still get the "Enter Parameter Value" window!
-I'm using a Requery on the Subform's control on the MainForm when the row is clicked.
-A MsgBox within the OnClick event, is giving me the value that the SQL statement should be using!
But the Subform doesn't seem to want to 'see' this value.

Any suggestions?

Thanks.