 <p>I had to DisableProxyCreation in the context configuration:</p> <pre><code>[OperationContract] [WebGet(UriTemplate = "")] public List&lt;ParentClass&gt; GetParents() { using (DBContext context = new DBContext()) { context.Configuration.ProxyCreationEnabled = false; List&lt;ParentClass&gt; parents = context.Parents .Include("Children") .ToList(); return playlists; } } </code></pre> <p>This worked out for me fine.</p>