seeing "Error: rlm_sql: Stop packet with zero session length"

Dear Sirs, we are running freeradius for VoIP accounting for Cisco AS5350 I see many-many records like Error: rlm_sql: Stop packet with zero session length. (user 'XXX', nas 'YYY') in radius.log I'm told that there's no error at Cisco, and that situation happens when callee doesn't accept call, that's why session length is equal to zero. Can status of that situation be reconsidered to "no error" ? I really don't want seeing the same message thousand times. Cheers, Ilia Chipitsine