How to avoid 'Input parameter ... failed to collect data' for empty input curve parameters

I have a custom component that can work on either 2 or 3 lists of curves as inputs, each of which is set up as a separate input parameter. However, it's not possible to set default values for Curve parameters, which forces the user to connect all three curve input parameters (even if only 2 are required) to avoid the message 'Input parameter ... failed to collect data'. How can I set up the curve inputs so that null values are valid? I'm currently registering these as curve parameters as below, and suspect the answer lies in using a different method for parameter registration.

protected override void RegisterInputParams(GH_Component.GH_InputParamManagerpManager)

{

pManager.Register_SurfaceParam(

"Reference Surface", "S", "Surface on which laths are to be generated", GH_ParamAccess.item);

pManager.Register_CurveParam(

"Surface curves 1", "Curves 1", "Set of curves across surface in first direction", GH_ParamAccess.list);

pManager.Register_CurveParam(

"Surface curves 2", "Curves 2", "Set of curves across surface in second direction", GH_ParamAccess.list);

pManager.Register_CurveParam(

"Surface Curves 3", "Curves 3", "Set of curves across surface in third direction", GH_ParamAccess.list);

pManager.Register_DoubleParam(

"Lath Offsets 1", "LO1", "Offset from surface to centreline of first layer", 0.0, GH_ParamAccess.item);

pManager.Register_DoubleParam(

"Lath Offsets 2", "LO2", "Offset from surface to centreline of second layer", 0.0, GH_ParamAccess.item);

pManager.Register_DoubleParam(

"Lath Offsets 3", "LO3", "Offset from surface to centreline of third layer", 0.0, GH_ParamAccess.item);

pManager.Register_IntegerParam(

"Seed Value (0, 1, 2)", "Seed", "Seed value for weave offsets (0 for no weave, 1 or 2 for weave)",0, GH_ParamAccess.item);

}

Thanks!

Alex