Невозможно выполнить операцию подстроки с чем-то, что не является строкой

У меня есть база данных с основными данными, и я пытаюсь сделать ее доступной для поиска. Он не работает и возвращает эту ошибку:

' невозможно выполнить операцию подстроки с чем-то, что не является строкой (lhs = (сущность: устройство; id: 0x8c63bc0 ; данные: {.......}

Вот мой код для создания ячеек.

- (NSManagedObjectContext *)managedObjectContext {
    NSManagedObjectContext *context = nil;
    id delegate = [[UIApplication sharedApplication] delegate];
    if ([delegate performSelector:@selector(managedObjectContext)]) {
        context = [delegate managedObjectContext];
    }
    return context;
}

- (id)initWithStyle:(UITableViewStyle)style
{
    self = [super initWithStyle:style];
    if (self) {
        // Custom initialization
    }
    return self;
}

- (void)viewDidLoad
{

    self.searchResults = [[NSArray alloc]init];

    [super viewDidLoad];

    // Uncomment the following line to preserve selection between presentations.
    // self.clearsSelectionOnViewWillAppear = NO;

    // Uncomment the following line to display an Edit button in the navigation bar for this view controller.
    // self.navigationItem.rightBarButtonItem = self.editButtonItem;
}

- (void)viewDidAppear:(BOOL)animated
{
    [super viewDidAppear:animated];

    // Fetch the devices from persistent data store
    NSManagedObjectContext *managedObjectContext = [self managedObjectContext];
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] initWithEntityName:@"Device"];
    self.devices = [[managedObjectContext executeFetchRequest:fetchRequest error:nil] mutableCopy];

    [self.tableView reloadData];
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

#pragma mark - Table view data source

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    // Return the number of sections.
    return 1;
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{

    if (tableView == self.searchDisplayController.searchResultsTableView) {
        return [self.searchResults count];
    }

    else
    {
        return [self.devices count];

    }

    // Return the number of rows in the section.
  //  return self.devices.count;
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *CellIdentifier = @"Cell";
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier forIndexPath:indexPath];

    if (tableView == self.searchDisplayController.searchResultsTableView) {

        NSManagedObject *device = [self.searchResults objectAtIndex:indexPath.row];
        [cell.textLabel setText:[NSString stringWithFormat:@"%@", [device valueForKey:@"text1"]]];
        [cell.detailTextLabel setText:[device valueForKey:@"text3"]];

    }

    else

    {

    // Configure the cell...
    NSManagedObject *device = [self.devices objectAtIndex:indexPath.row];
    [cell.textLabel setText:[NSString stringWithFormat:@"%@", [device valueForKey:@"text1"]]];
    [cell.detailTextLabel setText:[device valueForKey:@"text3"]];

    }

    return cell;
}


- (BOOL)tableView:(UITableView *)tableView canEditRowAtIndexPath:(NSIndexPath *)indexPath
{
    // Return NO if you do not want the specified item to be editable.
    return YES;
}


- (void)tableView:(UITableView *)tableView commitEditingStyle:(UITableViewCellEditingStyle)editingStyle forRowAtIndexPath:(NSIndexPath *)indexPath
{
    NSManagedObjectContext *context = [self managedObjectContext];

    if (editingStyle == UITableViewCellEditingStyleDelete) {
        // Delete object from database
        [context deleteObject:[self.devices objectAtIndex:indexPath.row]];

        NSError *error = nil;
        if (![context save:&error]) {
            NSLog(@"Can't Delete! %@ %@", error, [error localizedDescription]);
            return;
        }

        // Remove device from table view
        [self.devices removeObjectAtIndex:indexPath.row];
        [self.tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
    }
}



#pragma mark - Table view delegate

- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath
{
    // Navigation logic may go here. Create and push another view controller.
    /*
     <#DetailViewController#> *detailViewController = [[<#DetailViewController#> alloc] initWithNibName:@"<#Nib name#>" bundle:nil];
     // ...
     // Pass the selected object to the new view controller.
     [self.navigationController pushViewController:detailViewController animated:YES];
     */
}


#pragma mark - Segue

- (void)prepareForSegue:(UIStoryboardSegue *)segue sender:(id)sender
{
    if ([[segue identifier] isEqualToString:@"UpdateJob"]) {
        NSManagedObject *selectedDevice = [self.devices objectAtIndex:[[self.tableView indexPathForSelectedRow] row]];
        JobDetailViewController *destViewController = segue.destinationViewController;
        destViewController.device = selectedDevice;
    }
}

И код для запроса:

#pragma Search Methods

-(void)filterContentForSearchText:(NSString *)searchText scope:(NSString *)scope

{
    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"SELF beginswith[c] %@",searchText];
    self.searchResults = [self.devices filteredArrayUsingPredicate:predicate];



}

-(BOOL)searchDisplayController:(UISearchDisplayController *)controller shouldReloadTableForSearchString:(NSString *)searchString
{
    [self filterContentForSearchText:searchString scope:[[self.searchDisplayController.searchBar scopeButtonTitles] objectAtIndex:[self.searchDisplayController.searchBar selectedScopeButtonIndex]]];
    return YES;
}