diff --git a/install-stack.sh b/install-stack.sh
index c8caf4e..6284964 100755
--- a/install-stack.sh
+++ b/install-stack.sh
@@ -483,7 +483,7 @@ wait_healthy() {
   local id=$1 url=$2 max=${3:-40}
   local i=0
   while ! pct exec "$id" -- curl -sf --max-time 2 "$url" &>/dev/null; do
-    sleep 3; ((i++))
+    sleep 3; i=$((i+1))
     [[ $i -ge $max ]] && return 1
   done
   return 0
@@ -493,7 +493,7 @@ wait_pg() {
   local max=30 i=0
   while ! pct exec 100 -- bash -c \
     "docker exec hotel-manage-postgres pg_isready -U postgres" &>/dev/null; do
-    sleep 3; ((i++))
+    sleep 3; i=$((i+1))
     [[ $i -ge $max ]] && { msg_warn "Postgres not ready after 90s"; return 1; }
   done
 }
@@ -685,7 +685,7 @@ EOF
   # NPM admin API on port 81 — wait up to 60s
   local i=0
   while ! pct exec 103 -- curl -sf --max-time 3 "http://localhost:81/api/" &>/dev/null; do
-    sleep 3; ((i++)); [[ $i -ge 20 ]] && break
+    sleep 3; i=$((i+1)); [[ $i -ge 20 ]] && break
   done
   msg_ok "NPM running — admin UI at http://${NPM_LAN_IP}:81"
   msg_warn "NPM default login: admin@example.com / changeme  (change immediately!)"
@@ -774,7 +774,7 @@ configure_npm_proxy_hosts() {
 
   local i=0
   while ! curl -sf --max-time 3 "http://${NPM_LAN_IP}:81/api/" &>/dev/null; do
-    sleep 3; ((i++))
+    sleep 3; i=$((i+1))
     [[ $i -ge 30 ]] && { msg_warn "NPM API not responding — configure proxy hosts manually"; return; }
   done