Rayshader: adding a white background to a 2D plot

Question

I'm trying to add a white background to a 2D plot with the rayshader R package.

The approach I'm taking is to create an image with 255 values and then use the add_overlay function.

Here is the code I have:

Read in a tif file (which includes NA values surrounding the area of interest):

dem_sp <- terra::rast("dem_image_with_nas.tif")

Replace NAs with 255 to create white image

dem_sp_255 <- terra::subst(dem_sp, NA, 255) 

Convert raster to matrix

v_mat_nas <- raster_to_matrix(dem_sp)
v_mat_255 <- raster_to_matrix(dem_sp_255)

Create ambient shade object

ambmat <- ambient_shade(v_mat_nas, zscale = 50)

Create basemap:

  basemap <- v_mat_nas %>%
  add_overlay(height_shade(v_mat_255), alphalayer = 0.5) %>%
  sphere_shade(texture = "imhof2") %>%
  add_shadow(ambmat, max_darken = 0.6) 

This is the error:

Error in newarray[, , 1] <- image_overlay : 
  number of items to replace is not a multiple of replacement length

I can't find many consistent examples on now to add images with the add_overlay function and very little on adding a white background to a 2D plot. For example, I'm unclear on the syntax for adding 'hillshade' to the add_overlay function as I want add the white-coloured image as background. Maybe there is a better way to do this?

Thanks for your advice.