Abaqus warning of Negative Eigen value

I am doing a FE formulation of surface elasticity. I wrote UEL for surface elements in addition to abaqus volume elements. But surfaces may have negative stiffness (constitutive relation as per Morton Gurtin's formulation). So as a result I am getting a warning of "Negative eigen values". Can any body tell me how abaqus is solving for this system matrix (with few negative eigen values) in Static General method?