diff --git a/scripts/sync-plugin-content.sh b/scripts/sync-plugin-content.sh
index f54e477..6949ae5 100755
--- a/scripts/sync-plugin-content.sh
+++ b/scripts/sync-plugin-content.sh
@@ -116,7 +116,7 @@ fi
 # silently share (and corrupt) the same $name.log/$name.status/$name.checkcopy
 # scratch paths below.
 declare -a seen_names=()
-for plugin_dir in "${plugin_dirs[@]}"; do
+for plugin_dir in ${plugin_dirs[@]+"${plugin_dirs[@]}"}; do
   name="$(basename "${plugin_dir%/}")"
   for seen in ${seen_names[@]+"${seen_names[@]}"}; do
     if [[ "$seen" == "$name" ]]; then
@@ -242,7 +242,7 @@ sync_one() {
     cp -a "$plugin_dir/." "$pack_cwd/"
   fi
 
-  if ! (cd "$pack_cwd" && apm pack --format plugin "${force_flag[@]}" -o "$scratch") >"$pack_log" 2>&1; then
+  if ! (cd "$pack_cwd" && apm pack --format plugin "${force_flag[@]+"${force_flag[@]}"}" -o "$scratch") >"$pack_log" 2>&1; then
     echo "FAIL $plugin_dir: apm pack failed:" >&2
     sed 's/^/    /' "$pack_log" >&2
     rm -f "$pack_log"
@@ -284,7 +284,7 @@ sync_one() {
 # on macOS.
 JOBS_LIMIT="$(getconf _NPROCESSORS_ONLN 2>/dev/null || echo 4)"
 running=0
-for plugin_dir in "${plugin_dirs[@]}"; do
+for plugin_dir in ${plugin_dirs[@]+"${plugin_dirs[@]}"}; do
   name="$(basename "${plugin_dir%/}")"
   (sync_one "$plugin_dir" "$SCRATCH_ROOT/$name.status") >"$SCRATCH_ROOT/$name.log" 2>&1 &
   running=$((running + 1))
@@ -295,7 +295,7 @@ for plugin_dir in "${plugin_dirs[@]}"; do
 done
 wait
 
-for plugin_dir in "${plugin_dirs[@]}"; do
+for plugin_dir in ${plugin_dirs[@]+"${plugin_dirs[@]}"}; do
   name="$(basename "${plugin_dir%/}")"
   cat "$SCRATCH_ROOT/$name.log" >&2
   status="$(cat "$SCRATCH_ROOT/$name.status" 2>/dev/null || echo 1)"