diff --git a/deploy.sh b/deploy.sh
index eb0ef49..3a4162a 100644
--- a/deploy.sh
+++ b/deploy.sh
@@ -423,7 +423,7 @@ services_config_generation() {
   for service in "${SELECTED_SERVICES[@]}"; do
     [[ "${SELECTED_SERVICES_SUBDOMAIN+x:-false}" ]] && echo -e "  numbus.services.${service}.enable.subdomain = \"${SELECTED_SERVICES_SUBDOMAIN[${i}]}\";" >> ${CONFIGURATION_PATH}
     echo -e "  numbus.services.${service}.enable = true;" >> ${CONFIGURATION_PATH}
-    i=((i + 1))
+    i=$((i + 1))
   done
 
   if [[ "${SELECTED_SERVICES_SUBDOMAIN+x:-false}" && -n "$SELECTED_DNS_SERVICE" && -n "${SELECTED_SERVICES_SUBDOMAIN[${i}]}" ]]; then