Generating samples within an sf MULTIPOLYGON in r

I'm trying to generate a set of sample points within a set of islands. I generate the coastline data as follows:

library(osmdata)
bb <- getbb(place_name= "Shetland")
osm_box <- opq(bbox = bb) %>% 
  add_osm_feature(key = "natural", value = "coastline") %>%
  osmdata_sf()

And then I create an sf feature of MULTIPOLYGON:

library(sf)
coast <- osm_box$osm_lines %>% 
   st_union() %>% 
   st_line_merge() %>% 
   st_cast("MULTIPOLYGON") %>%
   st_transform(27700)

This seems to be ok. Now when I try to generate my sample points:

coast %>% st_sample(1000, type= "hexagonal")

I get one features of geometry type POINT instead of what I expect, 1000 features of POINT all located within the MULTIPOLYGON.

Geometry set for 1 feature 
geometry type:  POINT
dimension:      XY
bbox:           xmin: 396882.7 ymin: 1138217 xmax: 396882.7 ymax: 1138217
epsg (SRID):    27700
proj4string:    +proj=tmerc +lat_0=49 +lon_0=-2 +k=0.9996012717 +x_0=400000 +y_0=-100000 +ellps=airy +towgs84=446.448,-125.157,542.06,0.15,0.247,0.842,-20.489 +units=m +no_defs
POINT (396882.7 1138217)

Any ideas?!