AVAssetExportSession sporadically causes a black border

Question

I am trying to trim the video that I am taking from my iOS application and export it as mp4. The original video is recorded perfectly, but in 50% of cases when I crop the video, there is a black frame first.

Here is my code.

AVURLAsset *asset = [AVURLAsset URLAssetWithURL:videoURL options:@{AVURLAssetPreferPreciseDurationAndTimingKey:@YES}];

AVMutableComposition *composition = [AVMutableComposition composition];
AVMutableCompositionTrack *videoTrack = [composition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];

AVAssetTrack *assetTrack = [[asset tracksWithMediaType:AVMediaTypeVideo] firstObject];
CMTime duration = assetTrack.timeRange.duration;
[videoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, duration) ofTrack:assetTrack atTime:kCMTimeZero error:nil];

AVMutableVideoComposition *videoComposition = [AVMutableVideoComposition videoComposition];
videoComposition.frameDuration = CMTimeMake(1, 30.0);
videoComposition.renderSize = CGSizeMake(videoTrack.naturalSize.height, videoTrack.naturalSize.height);

AVMutableVideoCompositionLayerInstruction *transformer = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:videoTrack];
CGAffineTransform t2 = CGAffineTransformRotate(t1, M_PI_2);
[transformer setTransform:t2 atTime:kCMTimeZero];

AVMutableVideoCompositionInstruction *instruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
instruction.layerInstructions = @[transformer];
instruction.timeRange = CMTimeRangeMake(kCMTimeZero, duration);

videoComposition.instructions = @[instruction];

AVAssetExportSession *exporter = [[AVAssetExportSession alloc] initWithAsset:composition presetName:AVAssetExportPresetHighestQuality];
exporter.videoComposition = videoComposition;
exporter.outputURL = outputURL;
exporter.outputFileType = AVFileTypeMPEG4;

[exporter exportAsynchronouslyWithCompletionHandler:^{
    //Stuff
}];