Adding subtitles with fluent-ffmpeg

I'm trying to burn in subtitles into a video. This command works perfectly from command line: ffmpeg -i in.mp4 -vf subtitles=sub.srt:force_style='Fontsize=20' out.mp4

My code on server side doesn't seem to do much (it creates test.mp4 though)

ffmpeg('temp/subtitleVideos/qTWVbM5pkKms_pJbE8OAvH3N.mp4')
            .outputOptions(
                '-vf subtitles=temp/subtitleVideos/qTWVbM5pkKms_pJbE8OAvH3N.srt'
            )
            .on('error', function(err) {
                console.log('Error: ' + err.message);
            })
            .save(path + 'test.mp4');

and I get the following error: Error: ffmpeg exited with code 1: Error opening filters!