Wrong number of normal vectors.

I'm importing a geometry from a file reading the vertices of the figures. For each type I've wrote the correct indices, for example:

        public GeometryModel3D Generate3D()
        {

               GeometryModel3D My3DModel = new GeometryModel3D(); //Materials ommited

            List<Point3D> puntos = new List<Point3D>();
            List<int> indices = new List<int>();

                if (Elementos[i] is CTRIA3)
                {
                    Point3D p1 = Elementos[i].Grids[0].ToPoint();
                    Point3D p2 = Elementos[i].Grids[1].ToPoint();
                    Point3D p3 = Elementos[i].Grids[2].ToPoint();
                    puntos.Add(p1);
                    puntos.Add(p2);
                    puntos.Add(p3);

                    int n = puntos.Count;

                    indices.Add(n - 3);
                    indices.Add(n - 2);
                    indices.Add(n - 1);

                }

                if (Elementos[i] is CQUAD4)
                {
                    Point3D p1 = Elementos[i].Grids[0].ToPoint();
                    Point3D p2 = Elementos[i].Grids[1].ToPoint();
                    Point3D p3 = Elementos[i].Grids[2].ToPoint();
                    Point3D p4 = Elementos[i].Grids[3].ToPoint();
                    puntos.Add(p1);
                    puntos.Add(p2);
                    puntos.Add(p3);
                    puntos.Add(p4);

                    int n = puntos.Count;

                    indices.Add(n - 4);
                    indices.Add(n - 3);
                    indices.Add(n - 2);

                    indices.Add(n - 4);
                    indices.Add(n - 2);
                    indices.Add(n - 1);

                }
            //More geometric stuff

            Mesh3D mesh = new Mesh3D(puntos, indices);
            MeshBuilder mb = new MeshBuilder(false, false); //I've tried the default constructor too

            mb.AddCylinder(new Point3D(0, 0, 0), new Point3D(100, 100, 100), 5, 20);
            mb.Append(mesh.ToMeshGeometry3D()); //I've tried switching the order
            
            //My3DModel.Geometry = mesh.ToMeshGeometry3D();
            My3DModel.Geometry = mb.ToMesh();
            return My3DModel;
}

What I want is to use both, manually created figures and using MeshBuilder class (for tubes, cones etc). But if I use the append method to try than, even if I do not add any stuff with the MeshBuilder I get this exception:

An unhandled exception of type 'System.InvalidOperationException' occurred in HelixToolkit.Wpf.dll

Additional information: Wrong number of normal vectors.

What can I do to fix this situation? I know that there is a method called add triangles but It would be a bit difficult to implement for me right now.

This looks like a bug in Mesh3D or MeshBuilder. I don't think you need to use a Mesh3D here. Try to use a mb.Append(puntos, indices)