node-fluent-ffmpeg: Convert m4a to mp3 gives Error:{}

Here is my code (CoffeScript):

command = new FFmpeg { source: current_track.track_file_stream_url }
          .withNoVideo()
          .withAudioBitrate '128k'
          .withAudioChannels 2
          .withAudioFrequency 48000
          .withAudioQuality 5
          .fromFormat 'm4a'
          .toFormat 'mp3'
          .on 'progress', (progress) ->
            console.log progress.percent
          .on 'error', (err) ->
            console.log err
          .on 'end', () ->
            console.log 'Processing finished successfully'
          .saveToFile "#{current_track.track_file_stream_url}.mp3"

Any ideas?