[Freeswitch-users] mod_nibblebill question

I am using latest freeswitch and trying to test mod_nibblebill. I am trying to bill only b leg and it seems to cut the amount as per rates and heartbeat. But it does not restrict call if balance is 0 or in minus. I was hoping it will cut the calls the moment it reaches 0 session:execute("export", "nolocal:enable_heartbeat_events=60") session:execute("export", "nolocal:nibble_account=1") session:execute("export", "nolocal:nibble_increment=1") session:execute("export", "nolocal:nibble_rate=0.02") Is there anything I am doing wrong?