How do I save a list of nodes in my paramblock?

I've tried this:

ParamBlockDesc.AddParam(2, new object[] {"nodes", ParamType2.Tab, ParamBlock2Flags.VariableSize});

...and this:

ParamBlockDesc.AddParam(2, new object[] {"nodes", ParamType2.InodeTab, ParamBlock2Flags.VariableSize});

..but they both throw exceptions.