diff --git a/deploy/webmail.sh b/deploy/webmail.sh
index 9fc94b0..a77f358 100755
--- a/deploy/webmail.sh
+++ b/deploy/webmail.sh
@@ -363,8 +363,12 @@ cmd_up() {
 then re-run this command."
     fi
     detect_compose
+    # ${arr[@]+"${arr[@]}"} rather than plain "${arr[@]}": under `set -u`, bash
+    # 4.3 and older — which is what QuTS hero ships — treats expanding an EMPTY
+    # array as an unbound variable and aborts. The +-form expands to nothing at
+    # all when the array is empty, and is identical otherwise.
     # shellcheck disable=SC2086
-    compose "${profile[@]}" up -d "${build[@]}" $COMPOSE_WAIT
+    compose ${profile[@]+"${profile[@]}"} up -d ${build[@]+"${build[@]}"} $COMPOSE_WAIT
     health_gate || die "services did not come up healthy — check: ./deploy/webmail.sh logs"
     record_version "${IMAGE_TAG:-local}"
     grn "up. open http://<this-host>:${WEB_PORT:-8088}/"