Inserting an HTTP stream into a AVMutableComposition

I am trying to insert an AVURLAsset of a AVPlayerItem that states AVPlayerItemStatusReadyToPlay into an AVMutableComposition like this:

composition_ = [[AVMutableComposition alloc] init];
insertionPoint_ = kCMTimeZero;
item_ = [[AVPlayerItem playerItemWithURL:[NSURL URLWithString:@"http://devimages.apple.com/iphone/samples/bipbop/bipbopall.m3u8"]] retain];
[item_ addObserver:self forKeyPath:@"status" options:0 context:nil];
player_ = [[AVPlayer playerWithPlayerItem:item_] retain];
[player_ addObserver:self forKeyPath:@"currentItem.duration" options:0 context:nil];


/**
 * append a player-item to our composition 
 */
- (void)addItemToComposition:(AVPlayerItem *)item
{
    NSError *error = nil;
    VTRACE(@"item duration: %g", CMTimeGetSeconds(item.duration));
    if (![composition_ insertTimeRange:CMTimeRangeMake(kCMTimeZero, item.duration) 
                              ofAsset:item.asset
                               atTime:insertionPoint_ 
                                error:&error]) 
    {
        VTRACE(@"error: %@", error);
    }
}

/**
 * simplified value observer callback
 */
- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary *)change context:(void *)context
{
    if ([object isKindOfClass:[AVPlayerItem class]])
    {
        AVPlayerItem *item = (AVPlayerItem *)object;
        //playerItem status value changed?
        if ([keyPath isEqualToString:@"status"])
        {   //yes->
            switch(item.status)
            {
                case AVPlayerItemStatusFailed:
                    VTRACE(@"player item status failed");
                break;
                case AVPlayerItemStatusReadyToPlay:
                    VTRACE(@"player item status is ready to play");
                    [self addItemToComposition:player_.currentItem];
                break;
                case AVPlayerItemStatusUnknown:
                    VTRACE(@"player item status is unknown");
                break;
            }
        }
    }
    else if([object isKindOfClass:[AVPlayer class]])
    {
        if ([keyPath isEqualToString:@"currentItem.duration"])
        {   //yes->
            VTRACE(@"player item duration available");
        }
    }
}

Unfortunately, all I get is this lousy error message from the attempt to invoke AVMutableComposition insertTimeRange:ofAsset:atTime:error:

2011-11-29 22:24:59.446 XXX[13626:10703] -[XXX addItemToComposition:] -- error: Error Domain=AVFoundationErrorDomain Code=-11800 "The operation could not be completed" UserInfo=0x7699f30 {NSLocalizedFailureReason=An unknown error occurred (-12780), NSUnderlyingError=0xd124dd0 "The operation couldn’t be completed. (OSStatus error -12780.)", NSLocalizedDescription=The operation could not be completed}

What am I missing?

Why is it not possible to insert the item into the composition?

When inspecting the AVURLAsset of that AVPlayerItem I also noticed that the tracks array is allocated but empty. Could that be the reason, and if so, how do I get the AVURLAsset to properly hold valid tracks?

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