[c#] Why is this error, 'Sequence contains no elements', happening?

I am getting an Invalid Operation Exception, the stack is down below. I think it is because db.Responses.Where(y => y.ResponseId.Equals(item.ResponseId)).First(); is not returning any results. I checked the response data and the userResponseDetails has a ResponseId, I also just used a hard coded value. I also know that the statement that calls this one is adding the Responses row that this function should be calling. (This was working about a month ago and I don't remember changing anything that would break this)

[InvalidOperationException: Sequence contains no elements]
   System.Linq.Enumerable.First(IEnumerable`1 source) +269
   System.Data.Objects.ELinq.ObjectQueryProvider.<GetElementFunction>b__0(IEnumerable`1 sequence) +41
   System.Data.Objects.ELinq.ObjectQueryProvider.ExecuteSingle(IEnumerable`1 query, Expression queryRoot) +59
   System.Data.Objects.ELinq.ObjectQueryProvider.System.Linq.IQueryProvider.Execute(Expression expression) +133
   System.Data.Entity.Internal.Linq.DbQueryProvider.Execute(Expression expression) +87
   System.Linq.Queryable.First(IQueryable`1 source) +251
   InSight.Controllers.ForecasterController.userResponseDetails(List`1 userResponseDetails) +1039

Here is the offending code.

    [HttpPost]
    public JsonResult userResponseDetails(List<ResponseDetailsPartial> userResponseDetails)
    {

        foreach (ResponseDetailsPartial item in userResponseDetails)
        {
            ResponseDetails temp = new ResponseDetails();
            temp.ResponseId = item.ResponseId;
            temp.ResponseDetailVal = item.ResponseDetailVal;
            temp.QuestioChoicesId = item.QuestioChoicesId;
            temp.Response = db.Responses
                  .Where(y => y.ResponseId.Equals(item.ResponseId)).First();
            temp.QuestionChoice = db.QuestionChoices
                   .Where(x => x.QuestionChoicesId.Equals(item.QuestioChoicesId)).First();
          db.ResponseDetails.Add(temp);
        }
        db.SaveChanges();

        return Json(new { ResponseDetailsId = userResponseDetails }, JsonRequestBehavior.AllowGet);
    }

This is the AJAX that calls this specific action:

$.ajax({
         type: "POST",
         url: '/Forecaster/userResponseDetails/',
         data: JSON.stringify(rdetail),
         dataType: 'json',
         contentType: 'application/json',
     })

and this is rdetail after it has been strigified:

[{"ResponseId":118,"ResponseDetailVal":0.36,"QuestioChoicesId":null}] 

If this is the offending line:

db.Responses.Where(y => y.ResponseId.Equals(item.ResponseId)).First();

Then it's because there is no object in Responses for which the ResponseId == item.ResponseId, and you can't get the First() record if there are no matches.

Try this instead:

var response
  = db.Responses.Where(y => y.ResponseId.Equals(item.ResponseId)).FirstOrDefault();

if (response != null)
{
    // take some alternative action
}
else
    temp.Response = response;

The FirstOrDefault() extension returns an objects default value if no match is found. For most objects (other than primitive types), this is null.

In the following line.

temp.Response = db.Responses.Where(y => y.ResponseId.Equals(item.ResponseId)).First();

You are calling First but the collection returned from db.Responses.Where is empty.