diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 1500795..b31b635 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -12,7 +12,8 @@ permissions:
 jobs:
   test:
     name: Python ${{ matrix.python }}
-    runs-on: ubuntu-latest
+    # setup-python ships no 3.9 build for ubuntu-26.04 (ubuntu-latest); keep 3.9 on 24.04 while it stays supported.
+    runs-on: ${{ matrix.python == '3.9' && 'ubuntu-24.04' || 'ubuntu-latest' }}
     strategy:
       fail-fast: false
       matrix:
diff --git a/tests/test_behaviour_conformance.py b/tests/test_behaviour_conformance.py
index 3384f44..b737691 100644
--- a/tests/test_behaviour_conformance.py
+++ b/tests/test_behaviour_conformance.py
@@ -1030,6 +1030,11 @@ def run(self) -> None:
 def _eval_redis_layout(case: Mapping[str, Any]) -> None:
     """``redis_layout``: the three key-name cases (no broker) and the behavioural scenarios (Redis)."""
     if "layout" in case:
+        if case.get("name", "").startswith("encode-fails-") and redis_backend.connect()[1] == "emulated":
+            # fakeredis + lupa serialises inf/NaN without error where real lua-cjson raises, so the
+            # cjson.encode guard cannot be exercised on the emulator (see PopScriptWithoutRedisTest
+            # for the broker-free proof). CI runs these against a real redis:7 (BQ_REQUIRE_REDIS).
+            raise unittest.SkipTest("encode-fails-* needs a real Redis: the emulator's cjson masks the bug")
         _LayoutScenario(case).run()
     else:
         _eval_redis_key_names(case)
diff --git a/tests/test_redis_transport.py b/tests/test_redis_transport.py
index b8d0e6b..c172a4f 100644
--- a/tests/test_redis_transport.py
+++ b/tests/test_redis_transport.py
@@ -53,6 +53,10 @@
 SOURCE = Path(__file__).resolve().parent.parent / "src" / "babelqueue" / "redis_transport.py"
 
 
+def _reject_constant(name: str) -> None:
+    raise ValueError(f"{name} is not JSON (RFC 8259)")
+
+
 def assert_poison_record(case: unittest.TestCase, record: str, body: str, queue: str) -> None:
     """``record`` is the ADR-0035 poison DLQ record for ``body`` read from ``queue``."""
     parsed = json.loads(record)
@@ -66,7 +70,9 @@ def assert_poison_record(case: unittest.TestCase, record: str, body: str, queue:
     members = None
     if decoded is not None:
         try:
-            members = json.loads(decoded)
+            # ADR-0035 section 1: a strict RFC 8259 object is spliced; NaN / Infinity / -Infinity
+            # (which Python's json.loads accepts by default) make the body a wrapper case.
+            members = json.loads(decoded, parse_constant=_reject_constant)
         except ValueError:
             members = None
     if decoded is None:
@@ -255,7 +261,7 @@ class PopScriptWithoutRedisTest(unittest.TestCase):
     def _lua(self):
         try:
             from lupa import LuaRuntime
-        except ImportError:  # pragma: no cover - lupa ships with the test extras
+        except ImportError:  # pragma: no cover - lupa is optional (it comes with fakeredis[lua])
             self.skipTest("lupa is not installed")
         return LuaRuntime(unpack_returned_tuples=True)
 
@@ -294,6 +300,7 @@ def test_the_stub_models_the_bug_an_unguarded_encode_raises(self):
             "reserved = cjson.encode(decoded)",
         )
         self.assertNotEqual(unguarded, _POP)
+        self._lua()  # skips when lupa is not installed (the matrix jobs install no Redis extras)
         from lupa import LuaError
 
         with self.assertRaises(LuaError):
@@ -1101,7 +1108,8 @@ def test_undecodable_body_is_dead_lettered_raw_on_first_delivery(self):
         app = self.app(max_attempts=3, dead_letter=True)
         app.register(URN, lambda data, meta: None)
         self.assertEqual(app.consume(self.queue, max_messages=1, timeout=0), 1)
-        self.assertEqual(self.client.lrange(f"{self.queue}.dlq", 0, -1), [garbage])
+        (record,) = self.client.lrange(f"{self.queue}.dlq", 0, -1)
+        assert_poison_record(self, record, garbage, self.queue)  # ADR-0035 wrapper, not the raw body
         self.assertEqual(self.client.llen(self.processing), 0)
         self.assertEqual(self.client.zcard(self.deadlines), 0)
         self.assertEqual(self.client.llen(self.queue), 0)  # never re-queued for a retry
@@ -1111,7 +1119,8 @@ def test_undecodable_body_is_dead_lettered_with_the_dlq_off_too(self):
         app = self.app(max_attempts=3, dead_letter=False)
         app.register(URN, lambda data, meta: None)
         self.assertEqual(app.consume(self.queue, max_messages=1, timeout=0), 1)
-        self.assertEqual(self.client.lrange(f"{self.queue}.dlq", 0, -1), ["[1]"])
+        (record,) = self.client.lrange(f"{self.queue}.dlq", 0, -1)
+        assert_poison_record(self, record, "[1]", self.queue)
         self.assertEqual(self.client.llen(self.processing), 0)
 
     def test_publish_with_headers_stores_a_frame_and_pop_unframes(self):