Issue when adding text in liquidsoap

Question

Using liquidsoap to stream to facebook, however I have an issue where if i add text or image, i get the following error, any idea why

#!/usr/bin/liquidsoap
enc = %ffmpeg(
  format="flv",
  %audio.copy,
  %video.copy)
s = single("/home/liquidsoap/content/video/test.avi")
s = video.add_text(x=0, y=9, speed=0, size=40, "Hello world!", s)
s = mksafe(s)
key = "FB-763887877665745-0-AbyAWz_J6jQDRlQB"
url = "rtmps://live-api-s.facebook.com:443/rtmp/#{key}"
output.url(self_sync=true, url=url, enc, s)

Error message:

INFO: Loading Sdl_ttf, Target = linux INFO: Loading Sdl_image, Target = linux At /home/liquidsoap/config/radio.liq, line 11, char 41: output.url(self_sync=true, url=url, enc, s) Error 5: this value has type source(audio=?internal('A),...) (inferred at /usr/share/liquidsoap/2.1.4/libs/source.liq, line 6, char 2-75) but it should be a subtype of source(audio=ffmpeg.audio.copy,...)