AVAsset incorrect transform after adding watermark

I am trying to create a new video file by merging a video and image together (Like a watermark).

I can get it to work to a certain extent, however the video gets messed up unless I increase the width to equal the height.

I think there must be an issue with the transform somewhere.

This is how the video turns out:

If I set the width to the same as the height, the video looks fine (But with black bars).

Can anyone shed some light on this please?

Here is my code:

- (void)mergeVideoUrl:(NSURL *)videoURL andOverlayImage:(UIImage *)overlayImage
{
    // Create an AVURLAsset
    AVURLAsset* videoAsset = [[AVURLAsset alloc]initWithURL:videoURL options:nil];
    AVMutableComposition* mixComposition = [AVMutableComposition composition];

    AVMutableCompositionTrack *compositionVideoTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
    AVAssetTrack *clipVideoTrack = [[videoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];
    [compositionVideoTrack insertTimeRange:CMTimeRangeMake(kCMTimeZero, videoAsset.duration)
                                   ofTrack:clipVideoTrack
                                    atTime:kCMTimeZero
                                     error:nil];

    [compositionVideoTrack setPreferredTransform:[[[videoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] preferredTransform]];

    CGSize videoSize = [clipVideoTrack naturalSize];

    // Get our image layer
    CALayer *aLayer = [CALayer layer];
    aLayer.contents = (id)overlayImage.CGImage;
    aLayer.frame = CGRectMake(0, 0, videoSize.width, videoSize.height);
    aLayer.opacity = 1.0;

    // Sort the layer order
    CALayer *parentLayer = [CALayer layer];
    CALayer *videoLayer = [CALayer layer];
    parentLayer.frame = CGRectMake(0, 0, videoSize.width, videoSize.height);
    videoLayer.frame = CGRectMake(0, 0, videoSize.width, videoSize.height);
    [parentLayer addSublayer:videoLayer];
    [parentLayer addSublayer:aLayer];

    // Create the composition and add the instructions to insert the layer
    AVMutableVideoComposition* videoComp = [AVMutableVideoComposition videoComposition];
    videoComp.renderSize = videoSize;
    videoComp.frameDuration = CMTimeMake(1, 30);
    videoComp.animationTool = [AVVideoCompositionCoreAnimationTool      videoCompositionCoreAnimationToolWithPostProcessingAsVideoLayer:videoLayer inLayer:parentLayer];

    // Instruction
    AVMutableVideoCompositionInstruction *instruction = [AVMutableVideoCompositionInstruction videoCompositionInstruction];
    instruction.timeRange = CMTimeRangeMake(kCMTimeZero, [mixComposition duration]);
    AVAssetTrack *videoTrack = [[mixComposition tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0];
    AVMutableVideoCompositionLayerInstruction* layerInstruction = [AVMutableVideoCompositionLayerInstruction videoCompositionLayerInstructionWithAssetTrack:videoTrack];
    instruction.layerInstructions = [NSArray arrayWithObject:layerInstruction];
    videoComp.instructions = [NSArray arrayWithObject: instruction];

    // Export
    AVAssetExportSession *assetExport = [[AVAssetExportSession alloc] initWithAsset:mixComposition presetName:AVAssetExportPresetHighestQuality];
    assetExport.videoComposition = videoComp;

    NSString* videoName = @"mynewwatermarkedvideo.mov";

    NSString *exportPath = [NSTemporaryDirectory() stringByAppendingPathComponent:videoName];
    NSURL    *exportUrl = [NSURL fileURLWithPath:exportPath];

    if ([[NSFileManager defaultManager] fileExistsAtPath:exportPath])
    {
        [[NSFileManager defaultManager] removeItemAtPath:exportPath error:nil];
    }

    assetExport.outputFileType = AVFileTypeQuickTimeMovie;
    assetExport.outputURL = exportUrl;
    assetExport.shouldOptimizeForNetworkUse = YES;

    [assetExport exportAsynchronouslyWithCompletionHandler:^(void)
     {
         //FINALIZATION CODE HERE
         NSLog(@"DONE: %@", exportUrl);
     }];
}