How to resolve this error "Array element can not be nil"?

Below code generates error

Array element can not be nil

when I assign the value in aryResult[0]

NSMutableArray *tempResult = [[NSMutableArray alloc] initWithCapacity:2];
switch (habitObj.NoOfTimesPer)
{
    case 1://NO_OF_TIMES_PER_DAY
    {
        tempResult = [self getTotalNoOfDaysOrWeeks_createdDate:habitObj.strCreatedDt totalsOf:@"TOTAL_DAYS"];
    }
        break;
    case 2://NO_OF_TIMES_PER_WEEK
    {
        tempResult = [self getTotalNoOfDaysOrWeeks_createdDate:habitObj.strCreatedDt totalsOf:@"TOTAL_WEEKS"];
    }
        break;
    default:
        break;
}
aryResult[0]    = tempResult[0];
aryResult[3]    = tempResult[1];

The code below does not generate error:

[tempResult addObject:[NSNumber numberWithInt:0]];
[tempResult addObject:[NSNumber numberWithInt:1]];
aryResult[0]    = tempResult[0];
aryResult[3]    = tempResult[1];