How Can I Check For Duplicate Entries After Leaving A Field?

I'm pretty new to Access, so please overlook any stupid questions :)
I have a data entry form that has one field that cannot be a duplicate. So, in the table I have it Indexed with No duplicates.
The problem is that the form doesn't notify the user that they have entered duplicate information until they close the form. So the 10 minutes they spent filling it out is wasted.
So, after consulting various posts, I did a BeforeUpdate event with the following code:
(the field that must be unique is called "LS Number", in the form "Q and D" based on the table "Q and D Database")


Private Sub LS_Number_BeforeUpdate(Cancel As Integer)
On Error GoTo LS_Number_BeforeUpdate_Err

If (DLookup("[LS Number]", "Q and D Database", "[LS Number]=Forms![Q and D]![LS Number]")) Then
MsgBox "The LS Number you entered already exists. Enter a unique LS Number", vbInformation, "Duplicate LS Number"

End If

LS_Number_BeforeUpdate_Exit:
Exit Sub

LS_Number_BeforeUpdate_Err:
MsgBox Error$
Resume LS_Number_BeforeUpdate_Exit

End Sub


When I test this by entering a duplicate LS Number, it gives me the error "Type Mismatch." Unfortunately, I have no idea what this means, nor do I know how to fix it.

Please help, my head hurts!