PNG conversion with viridis color chart error

Question

I created a script where I have a working directory with several png images with a greyscale map in it. I want to convert the greyscale color chart to a viridis color chart to show to occurrences of species on it.

However when I run my code, the black pixels are also converted (=no data because of the sea) to a color so that the continent is not visible anymore. Therefore I inserted an command where pixels with a grayscale value less than 5 have to be white in the output file but now I got this error:

Error in img[img[, , 1] < 5, 1:3] <- c(255, 255, 255) : 
  incorrect number of subscripts on matrix

Conversion of greyscale to viridis whereby the sea has a value as well instead of white:

Script:

library(png)
library(viridisLite)

# Get list of PNG files in working directory
png_files <- list.files(pattern = "\\.png$")

# Loop through PNG files and convert to viridis color chart PNG
for (i in 1:length(png_files)) {
  in_file <- png_files[i]
  out_file <- gsub("\\.png$", "_viridis.png", in_file)

  # Read the PNG file
  img <- readPNG(in_file)

  # Convert pixels with grayscale value less than 5 to white
  img[img[, , 1] < 5, 1:3] <- c(255, 255, 255)

  # Convert grayscale values to viridis colors
  intmat <- 255 * img[, , 1] 
  virmat <- viridis(256)[intmat + 1]
  virmat <- c(substr(virmat, 2, 3), substr(virmat, 4, 5), 
              substr(virmat, 6, 7), substr(virmat, 8, 9))
  virmat <- as.numeric(as.hexmode(virmat))/255
  dim(virmat) <- c(dim(intmat), 4)

  # Write the viridis PNG file
  writePNG(virmat, out_file)
}