diff --git a/backup.sh b/backup.sh
index a9aae9f..bffe645 100644
--- a/backup.sh
+++ b/backup.sh
@@ -6,6 +6,15 @@ set -euo pipefail
 SRC="${1:?usage: backup.sh SRC DEST}"
 DEST="${2:?usage: backup.sh SRC DEST}"
 
+if [[ ! -d "$SRC" ]]; then
+  echo "backup.sh: source '$SRC' is not a directory" >&2
+  exit 1
+fi
+if [[ ! -d "$DEST" ]]; then
+  echo "backup.sh: destination '$DEST' does not exist (is the NAS mounted?)" >&2
+  exit 1
+fi
+
 stamp=$(date +%Y%m%d-%H%M)
 out="$DEST/backup-$stamp.tar.gz"