Addition of a new dof: warning result using eigs

Hi!

I have a system with 24 dof and I'm adding an extra dof as a result of the addition of a tuned mass damper.

My matrices are generalized matrices, not diagonal and the damper is implemented in those nodes I know it is interfering in all matrices (mass, stiffness and damping matrix), however when I calculate the eigenfrequencies it seems that the matrices are not well formed and there appears these two messages:

Warning: (A-sigma*B) has small reciprocal condition estimate: 0.000000 indicating that sigma is near an exact eigenvalue. The algorithm may not converge unless you try a new value for sigma.

and

Warning: Matrix is close to singular or badly scaled. Results may be inaccurate. RCOND = 3.079986e-018.

I have checked that the resultant matrices have no NaN or Inf...

Could you help me?

Thank you in advance

Laura