用STATA做GMM 的时候，用了命令 xtdpdsys  lnrealfdi, vce(robust) twostep endog( lnmobilephone lnrealgdppc lnedu lnlabourcost lnge lnpa
> vedroad hinterland lnbeds)
出现了以下错误note: hinterland dropped from dgmmiv() because of collinearity
note: D.hinterland dropped from lgmmiv() because of collinearity
     _xtdpd_putlevinst():  3301  subscript invalid
         _xtdpd_mkzlev():     -  function returned error
         _xtdpd_dowork():     -  function returned error
                 <istmt>:     -  function returned error
请问是怎么回事，请求高人解答，谢谢