DocumentDB ReplaceDocument Fails

In Azure DocumentDB using .NET SDK, I get the following error when calling ReplaceDocumentAsync:


"Errors":["The input content is invalid because the required properties - 'id; ' - are missing","The request payload is invalid. Ensure to provide a valid request payload."]


It's a blog post scenario, when a new comment is added, I get the document, add the comment and call ReplaceDocumentAsync. Here's how I do it:



string query = "SELECT * FROM Posts p WHERE p.id = 'some guid'";

var post = Client.CreateDocumentQuery<Post>(Collection.DocumentsLink, query)
.AsEnumerable().FirstOrDefault();

post.Comments.Add(comment);

Document doc = Client.CreateDocumentQuery(Collection.DocumentsLink)
.Where(d => d.Id == id)
.AsEnumerable()
.FirstOrDefault();

var document = await Client.ReplaceDocumentAsync(doc.SelfLink, item);


What am I doing wrong?