diff --git a/backend/main.py b/backend/main.py
index 623bf475..ce0add12 100644
--- a/backend/main.py
+++ b/backend/main.py
@@ -156,7 +156,15 @@ async def lifespan(app: FastAPI):
 
     from services.region_lookup import warm_borders
 
-    await run_in_threadpool(warm_borders)
+    try:
+        await run_in_threadpool(warm_borders)
+    except Exception:
+        # Same policy as prime_pipeline_at_startup above: a missing or corrupt
+        # borders file costs /api/towers its region detection until the next
+        # restart (classify_region still loads on demand and surfaces the real
+        # error there); raising would abort the lifespan and take the whole
+        # API down with it.
+        logging.exception("Warming the border polygons failed; region lookup will load on demand")
 
     # Restore persisted state before accepting connections
     restored = restore_snapshot()
diff --git a/backend/services/region_lookup.py b/backend/services/region_lookup.py
index ed43c124..9a188a8b 100644
--- a/backend/services/region_lookup.py
+++ b/backend/services/region_lookup.py
@@ -42,11 +42,18 @@ def _load_borders() -> None:
             return
         with open(_BORDERS_PATH) as f:
             data = json.load(f)
+        loaded: dict[str, BaseGeometry] = {}
         for feature in data["features"]:
             admin = feature["properties"].get("ADMIN")
             source = _ADMIN_TO_SOURCE.get(admin)
             if source is not None:
-                _geoms[source] = shape(feature["geometry"])
+                loaded[source] = shape(feature["geometry"])
+        # Published complete or not at all: the fast path above reads _geoms
+        # without the lock, so filling it feature-by-feature would let a
+        # concurrent caller see {"us"} mid-parse, skip the load, and silently
+        # misclassify a Canadian point as unsupported. dict.update runs under
+        # one GIL hold, so readers see the border set whole or empty.
+        _geoms.update(loaded)
 
 
 def warm_borders() -> None:
diff --git a/backend/tests/test_adsb_seed_backend.py b/backend/tests/test_adsb_seed_backend.py
index 724bc153..d0c5dd39 100644
--- a/backend/tests/test_adsb_seed_backend.py
+++ b/backend/tests/test_adsb_seed_backend.py
@@ -241,6 +241,10 @@ def _assert_derived(hexn: str, raw: dict) -> None:
     ("ground", {"alt_baro": "ground", "gs": None, "track": "unknown"}),
 ]
 
+# The sim push endpoint rejects non-transponder hexes (id_utils.is_transponder_hex),
+# so its writer fixtures must look like real 24-bit addresses, not readable labels.
+_SIM_CASE_HEX = {name: f"a1b2c{i}" for i, (name, _kin) in enumerate(_WRITE_CASES)}
+
 
 class TestDerivedFieldsAtWriteTime:
     """Every path that writes state.adsb_aircraft stores the SI-unit fields
@@ -284,7 +288,7 @@ async def test_sim_ingest_writer(self, name, kin):
         # what is under test, not the gate in front of it.
         from routes.sim_ingest import sim_push_adsb_positions
 
-        hexn = f"sim{name}"
+        hexn = _SIM_CASE_HEX[name]
         entry = {"hex": hexn, "lat": 33.9, "lon": -84.6, **kin}
         await sim_push_adsb_positions(body={"ts_ms": 4242, "aircraft": [entry]}, _key=None)
 
diff --git a/backend/tests/test_region_lookup_warm.py b/backend/tests/test_region_lookup_warm.py
index 9aa8e207..17a42e08 100644
--- a/backend/tests/test_region_lookup_warm.py
+++ b/backend/tests/test_region_lookup_warm.py
@@ -59,9 +59,7 @@ def test_a_warm_cache_is_never_re_parsed(self):
         _clear_cache()
         region_lookup.warm_borders()
 
-        with unittest.mock.patch.object(
-            region_lookup, "shape", side_effect=AssertionError("re-parsed after warm-up")
-        ):
+        with unittest.mock.patch.object(region_lookup, "shape", side_effect=AssertionError("re-parsed after warm-up")):
             assert region_lookup.classify_region(42.38708028093612, -71.24905416622781) == "us"
             assert region_lookup.classify_region(48.8566, 2.3522) is None