issue with health monitor instances

My output:

 ltm pool ns {
members {
    10.10.10.17:domain {
        address 10.10.10.17
        monitor udp
        session monitor-enabled
        state up
    }
    10.10.10.18:domain {
        address 10.10.10.17
        session monitor-enabled
        state up
    }
}
monitor dns_health_monitor

ltm node 10.10.10.17 {
address 10.10.10.17
description host_10.10.10.17
monitor none
session monitor-enabled
}
ltm node 10.10.10.18 {
address 10.10.10.18
description host_10.10.10.18
monitor none
session monitor-enabled