I cannot believe that it's still not reported, I tried to find such an issue but seems it was not reported yet.

1. zabbix_get -s localhost -k proc.num[,,,anything_you_would_want]
   1

Other checks wok as they should:

1. zabbix_get -s localhost -k 'agent.version'
   2.0.7

1. zabbix_get -s localhost -k proc.num[anything_you_would_want]
   0

1. zabbix_get -s localhost -k proc.num[mysqld]
   1

1. zabbix_get -s localhost -k proc.num[zabbix_agentd]
   6

Reproduced on agent versions: 1.8.17, 2.0.0, 2.0.7, 2.1.2 (current svn head)