Close The Form

When the user clicks to close the form the Before Update event runs, and has the following code to check that all the required fields are completed:

If IsNull(Me.ToxReceived) Then
stdresponse = (MsgBox("You must enter the toxicology received date before continuing." & Chr(13) & Chr(10) & Chr(13) & Chr(10) & _
"Do you want to do that now? Click YES to return to complete the form, click NO to undo the record", vbYesNo, "Missing Data"))

If stdresponse = vbYes Then
Me.ToxReceived.SetFocus

Else
Cancel = True
Me.Undo
Exit Sub
End If
End If

If the user clicks no, and the record is undone, how do I get the close form event to still continue?

D