NAV2009 - Web Services - How do I read a null key?

So, is there a way to read this record via a webservice?

I’m just not sure what value to pass in as the “Primary_Key” because the C# compiler will not let me use a (String.Empty) nor ("") nor (" "), nor (null). So, what value do I use to indicate I would like to read a record which uses a empty string as the primary key?