SetFocus Error On Tag Check

I'm using the following code (from GHudson) to check for required fields being filled in but it stops at a particular combo box and gives the error message Runtime error 2110 "can't move the focus to the control" although it works on previous combos which are set up in the same way.

Private Sub Close_Click()
'Key "Required" in the Tag property of the form object.

Dim ctl As Control
For Each ctl In Me
If ctl.tag = "Required" Then
If IsNull(ctl) Or ctl = "" Then
MsgBox "You must complete all required fields to continue. Your cursor wll now be set to the missed field", vbCritical, "Required Field"
ctl.SetFocus
Exit Sub
End If
End If
Next
Set ctl = Nothing

DoCmd.RunCommand acCmdClose

'If MsgBox("Have you selected the record type and filled in the other yellow fields? These are mandatory. If you haven't - click on 'No' to return to form", vbQuestion + vbYesNo, "Open Form?") = vbYes Then
'DoCmd.SetWarnings False
'DoCmd.RunCommand acCmdClose
'DoCmd.SetWarnings True
'End If

End Sub