Validation causing error when form is cleared

Private Sub txtEstStartDate_Exit(ByVal Cancel As MSForms.ReturnBoolean)
  If Not IsDate(txtEstStartDate) Then
      MsgBox "Input must be a date in the format: 'mm/yyyy'"
      Cancel = True
   Else
     txtEstStartDate = Format(txtEstStartDate, "mmm/yyyy")
   End If
       Me.txtEstStartDate.SetFocus
End Sub

This throws an error when I clear the form, the debugger goes to the setfocus line? how do I get around this?

thanks for any help