I have created a custom monitor to one of my ciscos in the OID i have set 1.3.6.1.4.1.9.10.19.1.1.4.0 and when I try to test it it is giving me a result like sysUpTime.6.1.4.1.9.10.19.1.1.4.0:NULL
altough if I do from the Linux Console i get a good result: