Adding RigidElement to model causes NaN in displacement

[epsi1on]

Describe the bug When a rigid element is added to the model, then after solve displacements are NaN

To Reproduce This is the code We've used and caused wrong result/ runtime error

var model = StructureGenerator.Generate3DTetrahedralElementGrid(3, 3, 70);

                var e = 210e9;

                foreach (var elm in model.Elements)
                {
                    if(elm is TetrahedronElement)
                    {
                        var tet = elm as TetrahedronElement;

                        tet.Material = new Materials.UniformIsotropicMaterial(e, 0.25);
                    }
                }

                var dx = model.Nodes.Max(i => i.Location.X) - model.Nodes.Min(i => i.Location.X);
                var dy = model.Nodes.Max(i => i.Location.Y) - model.Nodes.Min(i => i.Location.Y);
                var dz = model.Nodes.Max(i => i.Location.Z) - model.Nodes.Min(i => i.Location.Z);

                var l = dz;// model.Nodes.Max(i => i.Location.Z);

                var cnt = model.Nodes.Where(i => i.Location.Z == l);

                var f = 1e7;
                var I = dy * dx * dx * dx / 12;
                var rigid = new MpcElements.RigidElement_MPC() { UseForAllLoads = true };

                foreach (var node in cnt)
                {
                    node.Loads.Add(new NodalLoad(new Force(f / cnt.Count(), 0, 0, 0, 0, 0)));
                    rigid.Nodes.Add(node);
                }

                model.MpcElements.Add(rigid);
                model.Trace.Listeners.Add(new ConsoleTraceListener()); 
                model.Solve_MPC();

                var delta = f * l * l * l / (3 * e * I);

                var t = cnt.FirstOrDefault().GetNodalDisplacement();

                var ratio = delta / t.DX;

Expected behavior The displacements should not be Na