Swift Export MP4 using AVAssetExportSession - swift

Question

I am trying to export an mp4 file within the documents directory. I am downloading that file from a remote URL and them attempting the trim it and create a new mp4 from the existing. Below is the code I am using the the exception that I am getting. This approach is working for .mov files and remote videos but not mp4.
func trimVideo(sourceURL: URL, startTime: Double, endTime: Double) {
let fileManager = FileManager.default
let documentDirectory = fileManager.urls(for: .documentDirectory, in: .userDomainMask)[0]
let asset = AVAsset(url: sourceURL)
let fileName = UUID().uuidString + ".mp4"
var outputURL = documentDirectory.appendingPathComponent("output")
do {
try fileManager.createDirectory(at: outputURL, withIntermediateDirectories: true, attributes: nil)
outputURL = outputURL.appendingPathComponent(fileName)
}catch let error {
print(error)
}
//Remove existing file
try? fileManager.removeItem(at: outputURL)
guard let exportSession = AVAssetExportSession(asset: asset, presetName: AVAssetExportPreset640x480) else { return }
exportSession.outputURL = outputURL
exportSession.outputFileType = AVFileType.mp4
let timeRange = CMTimeRange(start: CMTime(seconds: startTime, preferredTimescale: 1000),
end: CMTime(seconds: endTime, preferredTimescale: 1000))
exportSession.timeRange = timeRange
exportSession.exportAsynchronously {
switch exportSession.status {
case .completed:
print(outputURL.absoluteString)
case .failed:
print("failed \(exportSession.error.debugDescription)")
case .cancelled:
print("cancelled \(exportSession.error.debugDescription)")
default: break
}
}
}
failed Optional(Error Domain=AVFoundationErrorDomain Code=-11800 "The operation could not be completed" UserInfo={NSUnderlyingError=0x600003f432a0 {Error Domain=NSOSStatusErrorDomain Code=-16979 "(null)"}, NSLocalizedFailureReason=An unknown error occurred (-16979), NSURL=PATH_TO_MY_FILE.mp4, NSLocalizedDescription=The operation could not be completed})

Answer 1

Surfacing Victor's own answer from his comment:
the problem was that I was not creating the url that I passed in with fileURLWithPath as follows. let fileURL = URL(fileURLWithPath: filePath)