我在做stata的系统GMM回归，输入命令是
xtdpdsys tfplp m2rb1 cost goods asset invc roe pgdp fip,lags(1) maxldep(1) twostep
结果出不来，stata提示错误是
variance-covariance matrix of the two-step estimatoris not full rank
    Two-step estimator is not available.  One-step estimator is available and variance-covariance matrix
    provides correct coverage.
有大神知道是为什么吗