Asterisk Monitoring by Nagios

I Am able monitor Asterisk with Nagios but some problem in status
i am executing this command

/usr/local/nagios/libexec/check_snmp -H localhost -C public -o astChanTypeChannels.5 -P 2c -l “Current open ZAP channels”

I am getting this output
Current open ZAP channels OK - 0 | ASTERISK-MIB::astChanTypeChannels.1=0
but if i stop the asterisk and execute the same command
i am getting this
Current open ZAP channels OK - No Such Object available on this agent at this OID |

did any one tell what i am doing wrong