diff --git a/goodwe_controller/app/main.py b/goodwe_controller/app/main.py
index 2b569aa..cb71db6 100644
--- a/goodwe_controller/app/main.py
+++ b/goodwe_controller/app/main.py
@@ -7564,10 +7564,8 @@ class Controller:
             # picked on its own. At N=1 both branches are `maints[0]` and this
             # is the shipped call, byte for byte.
             #
-            # A PER-PACK BUTTON IS A UI CARD, NOT THIS ONE. `web.py` posts a
-            # bare `maint_start` with no unit, and inventing a unit parameter
-            # here would ship half a surface: the page would still have one
-            # button and the API would have two meanings.
+            # This is the bare request (no unit). A request naming a battery
+            # was handled above by `_maint_unit_action()` (MAINT-11).
             _m = self._maint_button(now)
             for msg in _m.force_start(now):
                 self.log_event(msg, hint=messages.hint("maintenance",
diff --git a/goodwe_controller/test_maint11.py b/goodwe_controller/test_maint11.py
index 726ed70..102c420 100644
--- a/goodwe_controller/test_maint11.py
+++ b/goodwe_controller/test_maint11.py
@@ -306,6 +306,7 @@ rs = asyncio.run(serve(c, [
     {"action": "maint_abort", "unit": "inverter-b"},
     {"action": "maint_start", "unit": None},
     {"action": "maint_abort"},
+    {"action": "maint_start", "unit": ""},
 ]))
 check("start with a unit: 200 and the unit's answer",
       rs[0] == (200, {"ok": True, "result": {"unit": "inverter-b",
@@ -326,6 +327,9 @@ check("a null unit is the bare call: 200 and the old answer",
       rs[5] == (200, {"ok": True, "result": {"phase": DRAIN}}))
 check("a request with no unit: 200 and the old answer", rs[6] == (
     200, {"ok": True, "result": {"phase": IDLE}}))
+check("an empty unit is refused (400), not taken as the bare call",
+      rs[7][0] == 400 and rs[7][1]["code"] == "unit"
+      and phases(c) == [IDLE, IDLE])
 
 # --- the words --------------------------------------------------------------
 print("\nthe words")
@@ -358,6 +362,9 @@ check("the buttons are drawn only with two or more batteries",
 check("both buttons post the battery's name and ask first",
       'JSON.stringify({ action: what, unit: b.name }) });' in JS and 'ask1("start_dialog", "maint_start")' in JS
       and 'ask1("abort_dialog", "maint_abort")' in JS)
+check("a refusal is shown in the page's language, by its code",
+      'const own = j.code && t("batteries.cycle." + j.code);' in JS
+      and 'toast(own ? say(own.description' in JS)
 check("the old header buttons still post a bare request",
       'ask("maint_start", () => simple("maint_start"))' in JS
       and "body: JSON.stringify({ action: what }) });" in JS)