Warning: Matrix is singular to working precision.

[Boris Penaloza]

Warning: Matrix is singular to working precision. 
> In mips at 422
  In mipsopf_solver at 145
  In opf_execute at 106
  In opf at 225
  In runopf at 96
  In pqinput at 53 
 
I am getting this message error as I try to run a simulation. Does anybody know 
what it means??
 
thanks...