Can anyone help me with this code?

I have a button in a form and when I press this button I want a messagebox showing me the value from a vield in a specific table.

Dim db As Database
Dim qdf As QueryDef
Dim strSQL As String

strSQL = "SELECT BANKSALDO FROM BANKEN WHERE BANKNAAM = '" & BANKID.Value & "';"

Set db = CurrentDb
Set qdf = db.CreateQueryDef("test1", strSQL)
MsgBox (qdf("BANKSALDO"))

It's now in the sub and it doesn't work :'(
Do I have to put it in a function? I prefear it in the sub of the button.

Thx a lot for u help