Note: the rank of the differenced variance matrix (0) does not equal the number of coefficients
        being tested (20); be sure this is what you expect, or there may be problems computing the
        test.  Examine the output of your estimators for anything unexpected and possibly consider
        scaling your variables so that the coefficients are on a similar scale.

Test:  Ho:  difference in coefficients not systematic
                  chi2(0) = (b-B)'[(V_b-V_B)^(-1)](b-B)
                          =        0.00
                Prob>chi2 =           .
                (V_b-V_B is not positive definite)

检验时出现这个。。到底是什么原因。。。。急求。