Save UIImage to Photo Album with writeImageToSavedPhotosAlbum

I'm trying to save a UIImage to Photo Album. I've tried severl methods the last one is:

-(IBAction)captureLocalImage:(id)sender{

[photoCaptureButton setEnabled:NO];

// Save to assets library
ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];

[library writeImageToSavedPhotosAlbum: imageView.image.CGImage metadata:nil completionBlock:^(NSURL *assetURL, NSError *error2)
 {
     //             report_memory(@"After writing to library");
     if (error2) {
         NSLog(@"ERROR: the image failed to be written");
     }
     else {
         NSLog(@"PHOTO SAVED - assetURL: %@", assetURL);
     }

     runOnMainQueueWithoutDeadlocking(^{
         //                 report_memory(@"Operation completed");
         [photoCaptureButton setEnabled:YES];
     });
 }];  

}

imageView is a UIImageView which contain the image I want to save. On log I got "PHOTO SAVED - assetURL: (null)" and the photo doesn't save to library.

What am I doing wrong?

I'm using GPUImage, Apparently there is some kind of problem with the library itself.

The code above is absolutely working. If you want to read more about this issue:

GPUImagePicture with imageFromCurrentlyProcessedOutput doesn't get UIImage