When I use the following code it will give me a blank image?

-(void)generateImage
 {
     NSURL *url = [NSURL URLWithString:@"http://174.120.32.69/~mobadmin/b/MMAApp/videos/FitnessTrainingTammy.mp4"]; 
     AVURLAsset *asset = [[AVURLAsset alloc] initWithURL:url options:nil];
     AVAssetImageGenerator *generator = [[AVAssetImageGenerator alloc] initWithAsset:asset];
     generator.appliesPreferredTrackTransform=TRUE;
     [asset release];
     CMTime thumbTime = CMTimeMakeWithSeconds(0,30);
 
     AVAssetImageGeneratorCompletionHandler handler = ^(CMTime requestedTime, CGImageRef im, CMTime actualTime, AVAssetImageGeneratorResult result, NSError *error){
         if (result != AVAssetImageGeneratorSucceeded) {
             NSLog(@"couldn't generate thumbnail, error:%@", error);
         }
         [self.button setImage:[UIImage imageWithCGImage:im] forState:UIControlStateNormal];
         UIImage *thumbImg = [[UIImage imageWithCGImage:im] retain];
         [generator release];
     };
 
     CGSize maxSize = CGSizeMake(320, 180);
     generator.maximumSize = maxSize;
     [generator generateCGImagesAsynchronouslyForTimes:[NSArray arrayWithObject:[NSValue valueWithCMTime:thumbTime]] completionHandler:handler];
 }
 

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