Error in TreeView populating using Entity Framework

private void PopulateTreeView()
        {
using (var context = new DiagnosoftContext())
                        {
                            var q = (from i in context.Patients
                                     group i by i.BillDate into grpDate
                                     select grpDate);          

                            var query = q.ToList();

 foreach (var item in query)
                            {
                                TreeNode treeRoot = new TreeNode();
                                treeRoot.Nodes.Add(item.ToString());
                                treePatient.Nodes.Add(treeRoot);
                            }
}