InstanceId null when invoking .Net Service

I invoke a .NET Service i have imported into webmethods (with the microsoft package).

A .Net service has 2 output parameters: return & instanceid

The return parameter outputs the correct returnvalue from my .net service

The instanceid parameter however always has an output of null?

Am i missing something here :confused: Do i need to implement some kind of interface in my .net class?

Thx!