Plotting geom_sf over ggmap shape

Question

I'm trying to plot a map shapefile on top of a map downloaded using the ggmap package. However, I get a cryptic error message and a not very useful traceback:

Code:

map <- get_stamenmap( bbox = c(left = 3, bottom = 48, right = 3.5, top = 49), zoom = 12, maptype = "terrain")

transformed_sample = st_transform(rpg_sf %>% filter(commune %in% smallcomslist), crs=4326)

ggmap(map) + geom_sf(data=transformed_sample, mapping=aes(fill=commune),lwd=0) 

#Here transformed_sample is an object of class "sf" containing some outlines of villages in the area given by bbox.

#This returns:

ggmap(map) + 
   geom_sf(data=transformed_sample, mapping=aes(fill=commune),lwd=0) 

Coordinate system already present. Adding new coordinate system, which will replace the existing one.

Error in FUN(X[[i]], ...) : object 'lon' not found traceback()
10: FUN(X[[i]], ...)
9: lapply(aesthetics, eval_tidy, data = data, env = env)
8: f(..., self = self)
7: l$compute_aesthetics(d, plot)
6: f(l = layers[[i]], d = data[[i]])
5: by_layer(function(l, d) l$compute_aesthetics(d, plot))
4: ggplot_build.ggplot(x)
3: ggplot_build(x)
2: print.ggplot(x)
1: (function (x, ...)
UseMethod("print"))(x)

I have no idea why a "lon" is expected. Any ideas?