why i am getting 500 error on post method in web api ? it creates records in database but show 500 error

my error which i am getting "Message": "An error has occurred.", "ExceptionMessage": "UrlHelper.Link must not return null.", "ExceptionType": "System.InvalidOperationException",

[Route(UrlClass.createEmployee)]

    [HttpPost]
    public IHttpActionResult createEmployee(Employee employee)
    {
        if (!ModelState.IsValid)
        {
            return BadRequest(ModelState);
        }

        db.Employees.Add(employee);

        try
        {
            db.SaveChanges();
        }
        catch (DbUpdateException)
        {
            if (EmployeeExists(employee.Employee_ID))
            {
                return Conflict();

            }
            else
            {
                throw;
            }
        }

        return CreatedAtRoute("GetAction", new { id = employee.Employee_ID }, employee);
    }