Ajax JSON call to back-end function always triggers error response, even though function works correctly

Question

I have the following JSON Ajax call:

$.ajax({
    url: '@Url.Action("AddOrUpdateTimeEntry", "TimeEntries")',
    contentType: "application/json; charset=utf-8",
    type: "POST",
    dataType: "json",
    data: payload,
    success: function (data) {
        var id = data.TimeEntryID;
        $(this).parent().parent().parent().attr('id', id);
    }
    error: alert("Error Saving Data - changes not saved to database")
});

which calls this function:

public ActionResult AddOrUpdateTimeEntry(TimeEntryData ted)
{
int rowId = Convert.ToInt16(ted.RowId.Split('_')[1]);
// If RowId = 0, it's a new row so create, otherwise update
bool IsNewRow = rowId == 0;
int hours = ted.Hours;    
TimeEntry te;
if (IsNewRow)
{
    te = new TimeEntry();
    te.ClientID = ted.ClientID;
    te.TaskTypeID = ted.TaskTypeID;
}
else
{    
    te = (from t in db.TimeEntries
            where t.TimeEntryID == rowId
            select t).FirstOrDefault();
}   
switch (ted.DayOfWeek)
{
    case "Mon":
        te.MonHours = hours;
        break;
    ...
}   
if (IsNewRow) // New row, so create a new entry in database
{
    db.TimeEntries.Add(te);
}
db.SaveChanges(); 
var id = te.TimeEntryID;
return Json(new { TimeEntryID = id });
}

The function works, in that the database is updated correctly, but the error: function:

error: alert("Error Saving Data - changes not saved to database");

is always fired. (I assume that the row id is not updated either as per the success: function.)

Answer 1

Assign a function to error:

error: function() {
  alert("Error Saving Data - changes not saved to database");
}

Answer 2

Try returning an int rather than Json.

I find that a lot of the time this can sometimes happen when you mis-declare the datatype or return incorrect data.