TFHpple randomly crashes in iOS

My app is simple. It reads the stock symbol (AAPL) from Yahoo Finance and gets the price. Every half of a second it refreshes the stock price. Sometimes it crashes, can anyone help explain this to me, as to why it crashes. I get the error "unable to parse"

#import "ViewController.h"
#import "TFHpple.h"

@interface ViewController ()

@property UILabel *stockStymbol;
@property UILabel *stockPrice;

@end
//@"http://finance.yahoo.com/q?s=AAPL"
@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];

    _stockStymbol = [[UILabel alloc]init];
    _stockPrice   = [[UILabel alloc]init];


    [self parseStockSymbol];

    [NSTimer scheduledTimerWithTimeInterval:0.5f
                                     target:self
                                   selector:@selector(parseStockSymbol)
                                   userInfo:nil repeats:YES];
}


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

- (void) parseStockSymbol

{

    NSURL* baseURL = [NSURL URLWithString:@"http://finance.yahoo.com/q?s=AAPL"];
    NSData *data = [[NSData alloc]initWithContentsOfURL:baseURL];

    TFHpple *xpathParser = [[TFHpple alloc]initWithHTMLData:data];

    NSArray *elements  = [xpathParser searchWithXPathQuery:@"//span[@class='time_rtq_ticker']"];
    TFHppleElement *element = [elements objectAtIndex:0];

    _stockStymbol.text = @"AAPL";
    [_stockStymbol sizeToFit];
    _stockPrice.text = [[element firstChild]content];
    [_stockPrice sizeToFit];


    _stockPrice.frame = CGRectMake(100, 30, _stockPrice.bounds.size.width, _stockPrice.bounds.size.height);

      _stockStymbol.frame = CGRectMake(30, 30, _stockStymbol.bounds.size.width, _stockStymbol.bounds.size.height);


    [self.view addSubview:_stockStymbol];
    [self.view addSubview:_stockPrice];

}


@end