Unable to get OS from VIX API

I am trying to get OS Name by using VIX API in Java.

Syntax :

osName = vixVmHandle.readVariable("GuestOS", VixConstants.VIX_VM_CONFIG_RUNTIME_ONLY);

in above command I am getting null value in One ESX and in another ESX i am getting exact value.

I am unable to find such type of behavior of ESX server, same code is running successfully in one ESX and fail in another ESX.

Anybody know about this, please share your suggestions/ideas.

Thanks in advance