Making stringWithContentsOfURL asynchronous - Is it safe?

I attempted to make -[NSString stringWithContentsOfURL:encoding:error:] asynchronous, by running it a-synchronically from a background thread:

__block NSString *result;
dispatch_queue_t currentQueue = dispatch_get_current_queue();

void (^doneBlock)(void) = ^{
    printf("done! %s",[result UTF8String]);
};

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,
                                         (unsigned long)NULL), ^(void) {
    result = [NSString stringWithContentsOfURL:[NSURL URLWithString:@"http://www.google.com/"] encoding:NSUTF8StringEncoding error:nil];
    dispatch_sync(currentQueue, ^{
        doneBlock();
    });
});

Its working fine, and most importantly, its asynchronous.

My question is if it's safe to do this, or could there be any threading problems etc.?

Thanks in advance :)