以下是插入的默认路线和帖子.当我调用帖子时,记录在数据库中完美创建,但是“CreatedAtRoute”调用返回500错误,说明：

ExceptionMessage: “UrlHelper.Link must not return null.”
ExceptionType: “System.InvalidOperationException”

为什么会收到这个错误？

[RoutePrefix("api/casenotes")]
    public class CasenoteController : ApiController...



// POST api/Casenote
[Route("")]
[ResponseType(typeof(client_admission_casenote))]
    public async Task<IHttpActionResult> Postclient_admission_casenote   (client_admission_casenote client_admission_casenote)
{

  Request.GetRequestContext().IncludeErrorDetail = true;

  if (!ModelState.IsValid)
  {
      return BadRequest(ModelState);
  }

  db.client_admission_casenote.Add(client_admission_casenote);
  await db.SaveChangesAsync();

  return CreatedAtRoute("DefaultApi",new { id = client_admission_casenote.casenote_id },client_admission_casenote);
    }