[R] Error when running coeftest in plm

[Hien Nguyen]

Dear R-helpers,

I am using plm package to run panel data analysis for different models 
with different dependent variables. After running the PLM, I run the 
COEFTEST(plm1, vcovBK) to get the panel corrected standard errors. I got 
the PCSEs for some of the models but some tests produce error message.  
In one of the COEFTEST, the error message is:

Error in solve.default(crossprod(demX)) :
   system is computationally singular: reciprocal condition number = 
1.404e-017

What should I do to fix it and get the PCSEs?

Thanks a lot,

Hien Nguyen