Query Won't Accept Multiselect LB Criteria

Hello,

I've scoured the dozen archived threads that relate to my challenge, but I still can't quite overcome it.

I have an aggregate query that runs but returns no values.

This is how I've tried to set things up:
A user makes a multiple selection in a list box on a form.
The multiple selection gets strung into an SQL that I feed to a hidden control on the form.
Whatever is in the hidden control then becomes the Where clause that I reference in a Criteria cell in my query.
The query is run when the user presses a command button.

Can anybody tell what's wrong with my code? Thanks very much in advance!

Private Sub cmdTractSelect_Click()
On Error GoTo Err_cmdTractSelect_Click

'Declare variables
Dim db As Database
Dim tbl As TableDef
Dim fld As Field
Dim qry As QueryDef
Dim frm As Form
Dim ctl As Control
Dim varItem As Variant
Dim strSQL As String

'Assign values to the variables
Set db = CurrentDb()
Set tbl = db.TableDefs("City")
Set fld = tbl.Fields("Tract")
Set qry = db.QueryDefs("Param")
Set frm = Forms!frmTractSelect
Set ctl = frm!lstTract
strSQL = "[Tract]="

'Grab the list-box selection and string it into SQL
For Each varItem In ctl.ItemsSelected
strSQL = strSQL & ctl.ItemData(varItem) & " Or [Tract]="
Next varItem

'Make sure the user selected at least one tract
If Len(strSQL) = 0 Then
MsgBox "You didn't select anything" _
, vbExclamation, "Nothing to find!"
Exit Sub
End If

'Trim the SQL
strSQL = Left$(strSQL, Len(strSQL) - 12)

'Insert the SQL into a hidden control that will feed_
'the criteria of the Tract field
txtWhere.Value = strSQL

'Run the query
DoCmd.OpenQuery ("Param")

Exit_cmdTractSelect_Click:
Exit Sub

Err_cmdTractSelect_Click:
MsgBox Err.Description
Resume Exit_cmdTractSelect_Click

End Sub