I think the issue is those aren't integers, they are ffmpeg expressions evaluating to integers. See e.g. this example from the ffmpeg documentation:

  Draw a 2-pixel red 2.40:1 mask:
  drawbox=x=-t:y=0.5*(ih-iw/2.4)-t:w=iw+t*2:h=iw/2.4+t*2:t=2:c=red

Implementing type checking on this correctly would require something like a DSL.