我近日对设定的面板数据模型进行FE和RE的hausman检验时，系统出现以下提示： the rank of the differenced variance matrix (4) does not equal the number of coefficients being tested (6); 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.
请问这是怎么回事？