vmrun.exe readvariable returns null

i have installed vmware VIX on windows 7 64bit and want to connect and read some virtual machine variables such as net.adapters, but it returns null.

my command is :

vmrun.exe -h myhost.com -U myusername -P mypass readVariable "vmpath.vmx" guestVar net.adapters

but it always returns null

but this command returns vm's ip correctly :

vmrun.exe -h myhost.com -U myusername -P mypass readVariable "vmpath.vmx" guestVar ip as VMTools is installed on vm

note that my user has full access on vmware server host.