copyNextSampleBuffer returns NULL when reading two-video-track movie second time

Question

I have a movie with two video tracks. I need to read frames from the two tracks interleaved - that is, one frame from track 1, then one frame from track 2, then from track 1 and so on. I'm using an AVAsset with two AVAssetReaders to read the frames and it works; however, if I deallocate everything and try a second time, copyNextSampleBuffer returns null on the first frame, and the error is -11829 (Cannot Open). This doesn't happen if I try a single-track movie, or if I try reading just a single track from the same movie.

This is my stripped-down code:

NSURL *url = /* url to movie */;
NSError *localError = nil;
AVAssetReader *assetReader[2];
AVAssetReaderOutput *assetReaderOutput[2];
NSDictionary *assetOptions = [NSDictionary dictionaryWithObject:[NSNumber numberWithBool:YES] forKey:AVURLAssetPreferPreciseDurationAndTimingKey];
NSDictionary *decompressionVideoSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                                                [NSNumber numberWithUnsignedInt:kCVPixelFormatType_32BGRA], (id)kCVPixelBufferPixelFormatTypeKey,
                                                [NSDictionary dictionary], (id)kCVPixelBufferIOSurfacePropertiesKey,
                                                nil];
for (int loop = 0; loop < 2; ++loop)
{
    AVAsset *asset = [AVURLAsset URLAssetWithURL:url options:assetOptions];
    float durationInSeconds = CMTimeGetSeconds(asset.duration);
    NSArray *videoTracks = [asset tracksWithMediaType:AVMediaTypeVideo];
    NSUInteger numTracks = [videoTracks count];
    float framesPerSecond = [videoTracks[0] nominalFrameRate];
    int numFrames = (int)(durationInSeconds * framesPerSecond);
    for (NSUInteger trackNum = 0; trackNum < numTracks; ++trackNum)
    {
        assetReader[trackNum] = [[AVAssetReader alloc] initWithAsset:asset error:&localError];
        assetReaderOutput[trackNum] = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:videoTracks[trackNum] outputSettings:decompressionVideoSettings];
        [assetReader[trackNum] addOutput:assetReaderOutput[trackNum]];
        [assetReader[trackNum] startReading];
    }

    for (int frameNum = 0; frameNum < numFrames; ++frameNum)
    {
        for (int trackNum = 0; trackNum < numTracks; ++trackNum)
        {
            CMSampleBufferRef sampleBuffer = [assetReaderOutput[trackNum] copyNextSampleBuffer];
            if (sampleBuffer == NULL)
            {
                NSError *error = [assetReader[trackNum] error];
                /* Handle error */
            }
            else
            {
                CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
                if (imageBuffer && (CFGetTypeID(imageBuffer) == CVPixelBufferGetTypeID()))
                {
                    CVPixelBufferRef pixelBuffer = (CVPixelBufferRef)imageBuffer;
                    CVPixelBufferLockBaseAddress(pixelBuffer, kCVPixelBufferLock_ReadOnly);
                    void* ptr = CVPixelBufferGetBaseAddress(pixelBuffer);
                    CVPixelBufferUnlockBaseAddress(pixelBuffer, kCVPixelBufferLock_ReadOnly);
                    /* Do something with ptr */
                }
                CFRelease(sampleBuffer);
                sampleBuffer = NULL;
            }
        }
    }

    [asset release];
    asset = nil;
    for (int trackNum = 0; trackNum < numTracks; ++trackNum)
    {
        [assetReader[trackNum] release];
        assetReader[trackNum] = nil;
        [assetReaderOutput[trackNum] release];
        assetReaderOutput[trackNum] = nil;
    }

    int userInput;
    scanf("%i", &userInput);
}

So on the second iteration of the outer loop, I get to the Handle error code on the first frame of the first track. Just to be clear, this does not happen if, for example, I replace the line

NSUInteger numTracks = [videoTracks count];

with

NSUInteger numTracks = 1;

Any ideas? This might be connected to this (yet unanswered) question.