Form Field Validation - "Cancel = True" Problem

I'm calling this code on my Form's BeforeUpdate Event which I understand should only run if the form is dirty. In this example, the code should check to see if 4 fields ("txtField1", "txtField2", txtField3", "txtField4") have data present, if not highlight them yellow.

Function Msg()
Dim ctl As Control
Dim strFields As String
Dim strControl As String
Dim intCounter As Integer
Dim blnEmpty As Boolean

For Each ctl In Me.Controls
blnEmpty = False
If ctl.ControlType = acComboBox Or acTextBox Then
If ctl.Visible And Len(ctl.Tag) > 0 Then
Select Case ctl.Tag
Case "txtField1", "txtField2", txtField3", "txtField4"
If IsNull(ctl.Value) Then
blnEmpty = True
ctl.BackColor = vbYellow
Else
ctl.BackColor = vbWhite
End If
Case Else
If ctl.Value = 0 Then
blnEmpty = True
ctl.BackColor = vbYellow
Else
ctl.BackColor = vbWhite
End If
End Select
If blnEmpty Then _
strFields = strFields & ctl.Tag & vbCrLf
If blnEmpty Then _
If Len(strControl) = 0 Then _
strControl = ctl.Name
End If
End If
Next

If Len(strFields) > 0 Then
Cancel = True
MsgBox "You have not completed all data fields, " & _
"please enter data in the following fields:" & vbCrLf & strFields, _
vbExclamation, Me.Caption
Me(strControl).SetFocus
Me(strControl).BackColor = vbWhite
Exit Function
End If
End Function

However, it keeps falling over at:
Cancel = True saying "Compile Error: Variable Not Defined"

I want to be able to run this code from a "Close Form button" to ensure that either the record is not created unless the fields listed are filled in or the form closure is cancelled so that the user can complete the missing data.

I've seen varients of this code on a couple of Access forums but can't find a solution for the "Cancel = True" issue.

Am I doing something wrong?