this derivation always fails for me with nix-build

always-fails.nix

let
  pkgs = import <nixpkgs> {};
  name = "psc-package2nix-prefetch-${dep}-${version}";

  dep = "simple-json";
  repo = "https://github.com/justinwoo/purescript-simple-json.git";
  version = "v4.0.0";

in pkgs.runCommand name {
  buildInputs = with pkgs; [
    nix-prefetch-git
    jq
  ];
} ''
  RESULT=$(nix-prefetch-git "${repo}" --rev "${version}" --quiet | jq '.sha256' -r)
  if [ $RESULT == '0sjjj9z1dhilhpc8pq4154czrb79z9cm044jvn75kxcjv6v5l2m5' ]; then
    echo "Prefetched revision was empty: ${repo} ${version}";
    exit 1;
  else
    echo $RESULT > $out;
  fi
''

# always fails with 0sjjj9z1dhilhpc8pq4154czrb79z9cm044jvn75kxcjv6v5l2m5, an empty fetched result