How can I make a polygon overlay transparent in R MapBoxAPI?

Question

I am using the mapboxapi package in R to save static aerial images

I want to see the outline of the polygon on a static image, with the fill being entirely transparent. Setting overlay_style$fill-opacity to 0 is not working. What am I doing wrong?

#make a polygon showing area you want to view in MapBox
lon = c(-128.6816, -128.6812)
lat = c(54.65256, 54.65292)
poly_coord_df = data.frame(lon, lat)
poly = poly_coord_df %>% 
  st_as_sf(coords = c("lon", "lat"), crs = 32611) %>% 
  st_bbox() %>% 
  st_as_sfc()

#make a list with desired overlay_style
o_style = list()
o_style$'stroke' = '#ff3300'
o_style$'fill' = '#ff3300 '
o_style$'fill-opacity' = 0 #setting fill-opacity to 0 is not working

#make the map
map = static_mapbox(
    style_id = 'satellite-v9',
    username = "mapbox",
    overlay_sf = poly,
    overlay_style = o_style,
    width = 1000,
    height = 800
  )
map