SNMP protocol in Logic Machine

[DGrandes]

Hi, my script still doesn´t work (only sometimes). I get the same error: 

Resident script:45: netsnmp: Unknown host ("MyIP")
stack traceback:
 [C]: in function 'assert'

Code:

require "snmp"

local IP_NAS = "MyIP"
local OID_NAS = {
 {'38/2/1',".1.3.6.1.4.1.24681.1.2.1.0"}, --Uso de la CPU
 {'38/2/2',".1.3.6.1.4.1.24681.1.2.2.0"}, --Memoria total del sistema
 {'38/2/3',".1.3.6.1.4.1.24681.1.2.3.0"}, --Memoria libre
 {'38/2/4',".1.3.6.1.4.1.24681.1.2.4.0"}, --Tiempo en linea
}

ups1, err = snmp.open{
 version = snmp.SNMPv2,
 community = "public",
 port = 161,
 peer = IP_NAS,
}

assert(ups1, err)

for i = 1, #OID_NAS, 1 do
    vbind, err = ups1:get(OID_NAS[i][2])
    grp.update(OID_NAS[i][1], vbind.value)  
end

ups1:close()

Any idea?

Thanks