GetData<double>(int, ref double)' has some invalid argument

hi

what on earth is the problem here:

pManager.AddPointParameter("bound", "bound\border", "", GH_ParamAccess.list);
pManager.AddPointParameter("inner", "inner points", "", GH_ParamAccess.list);
pManager.AddNumberParameter("Connectivity", "c", "", GH_ParamAccess.item, 0);

Param_Point param0 = (Param_Point)pManager[0];
Param_Point param1 = (Param_Point)pManager[1];
Param_Number param2 = (Param_Number)pManager[2];

}

protected override void RegisterOutputParams(GH_OutputParamManager pManager)
{
pManager.AddLineParameter("Lines of structure geometry", "Line", "", GH_ParamAccess.list);
pManager.AddPointParameter("Joints in structure - in the order of DOF", "joints", "", GH_ParamAccess.list);

}

public override GH_Exposure Exposure
{
get { return GH_Exposure.primary; }
}

protected override void SolveInstance(IGH_DataAccess DA)
{
// Declare local variables to store input lists.
List<Point3d> bound = new List<Point3d>();
List<Point3d> inner = new List<Point3d>();
double conn = 0;

// Since none of the inputs are optional, we can abort as soon as we have missing data.
if (!DA.GetDataList(0, bound)) return;
if (!DA.GetDataList(1, inner)) return;
if (!DA.GetData(2, conn)) return;

I get the error in the title..

thank you