SAC utlity on named instance (Clustered)

Hi All, 

I need to run a command similar to below on a named instance on a Clustered server:

sac out outputfile –S servername -I Instance2\Instance2

But I get an error saying:
Instance2\Instance2 instance not found on computer servername .

I can run the command fine on a standalone default instance. I guess, there is some mistake in my command about the instance name..

can someone help urgently????

any help is highly appreciated!!..

Thanks, 
Mandar