diff --git a/src/cipsi/selection.irp.f b/src/cipsi/selection.irp.f
index 6813b5a1..9390a9bf 100644
--- a/src/cipsi/selection.irp.f
+++ b/src/cipsi/selection.irp.f
@@ -789,7 +789,8 @@ subroutine fill_buffer_double(i_generator, sp, h1, h2, bannedOrb, banned, fock_d
       ! Gram-Schmidt using input overlap matrix
       do istate=1,N_states
         do jstate=1,istate-1
-          coef(istate) = coef(istate) - pt2_overlap(istate,jstate)/(1.d0+pt2_overlap(jstate,jstate)) * coef(jstate)
+          if ( (pt2_overlap(istate,jstate) == 0.d0).or.(pt2_overlap(jstate,jstate) == 0.d0) ) cycle
+          coef(istate) = coef(istate) - pt2_overlap(istate,jstate)/pt2_overlap(jstate,jstate) * coef(jstate)
         enddo
       enddo