在做聚类稳健标准差时，stata出现了这个提示：

Warning: estimated covariance matrix of moment conditions not of full rank.
         model tests should be interpreted with caution.
Possible causes:
         number of clusters insufficient to calculate robust covariance matrix
         singleton dummy variable (dummy with one 1 and N-1 0s or vice versa)
partial option may address problem.


求助大神。