Problem With Checking For Duplicates

I have a table that stores jobs. The primary key field is JOBID which will correspond to company invoice #'s. Because the Invoice #'s are generated by a different system, I don't not have this as an autonumber field, I want to control it programatically when the user enters a value into the JobID textbox.

At first, I was using DCOUNT on the BeforeUpdate event of the form to check if there was already a matching JobID in the table, and if so, cancelling the update.

intInvoiceCheck = DCount("[JobID]", "jobs", "[JobID]=" & Me.JobID)

If intInvoiceCheck > 0 Then
Response = MsgBox("You have entered an Invoice # that is already used." & _
Chr(10) & Chr(13) & Chr(10) & Chr(13) & _
"If this is a secondary invoice considering adding 'a' or 's' to make it unique." & _
Chr(10) & Chr(13) & _
"For example, " & Me.JobID & "a, " & Me.JobID & "s...", vbOKOnly, "Duplicate Invoice")
Me.JobID.SetFocus
Cancel = True
Exit Sub
End If

However, if the user is changing say a jobsite address, they can't, because it checks and sees that intInvoiceCheck = 1 and won't continue.

I tried moving my routine to the BeforeUpate event of the JobID textbox like this (this is not the final code, only what I testing with)

'Check that the entered Invoice Number value is unique
Dim intInvoiceCheck As Integer
Dim Response

intInvoiceCheck = DCount("[JobID]", "jobs", "[JobID]=" & Me.JobID.Value)

If intInvoiceCheck > 0 Then

MsgBox ("Dupe")
Cancel = True
Me.JobID.Undo

End If

However, I getthis long error message saying I have changed the validation rule of the field without first checking to see if it's valid....

I can change the to Me.Undo, which works fine, but I don't want to undo the whole entry, only the JobID and set focus to it, and make the user enter a new one.

Can anyone help me out with this?

Thanks,
Hammy