cover faces 出错！

用 cover faces时，报错：    [error] Acis error: "Fail to cover sheet.. Acis Error 25000 - edge circuit is not connected " (7:48:36 下午  十一月 09, 2009)
    [error] Acis error: "Fail to cover surfaces." (7:48:36 下午  十一月 09, 2009)
    [error] Acis error: "Fail to cover sheet.. Acis Error 25000 - edge circuit is not connected " (7:48:36 下午  十一月 09, 2009)
  但是，我画的各面应该是闭合的啊……无助中。