diff --git a/packages/cultcache-rs/tests/union_parity.rs b/packages/cultcache-rs/tests/union_parity.rs
index 11048577..75154f79 100644
--- a/packages/cultcache-rs/tests/union_parity.rs
+++ b/packages/cultcache-rs/tests/union_parity.rs
@@ -95,6 +95,13 @@ enum Outcome {
     Ceiling(IdArm),
 }
 
+// The codec's unit arm (no members, written [key, []]), kept apart from the golden: a document list's arms carry ids.
+#[derive(Clone, Debug, PartialEq, CultUnion)]
+enum UnitOutcome {
+    #[cultcache(key = 2)]
+    Denied,
+}
+
 #[derive(Clone, Debug, PartialEq, CultSlots)]
 struct PendingArm {
     #[cultcache(key = 0)]
@@ -241,6 +248,12 @@ fn union_surface_decodes_and_reencodes() {
     assert_eq!(rmp_serde::to_vec(&value).unwrap(), payload);
 }
 
+#[test]
+fn a_unit_arm_is_a_key_and_an_empty_slots_array() {
+    assert_eq!(rmp_serde::to_vec(&UnitOutcome::Denied).unwrap(), [0x92, 0x02, 0x90]);
+    assert_eq!(rmp_serde::from_slice::<UnitOutcome>(&[0x92, 0x02, 0x90]).unwrap(), UnitOutcome::Denied);
+}
+
 #[test]
 fn an_id_only_arm_is_a_key_and_a_one_slot_array() {
     let mut bytes = vec![0x92, 0x02, 0x91, 0xac];
@@ -307,8 +320,8 @@ fn a_union_that_is_not_a_key_and_slots_pair_is_refused_naming_the_enum() {
         ("a unit arm with slots", rmp_serde::to_vec(&(2u64, ("CANARY-slot",))).unwrap()),
     ];
     for (label, bytes) in cases {
-        let message = rmp_serde::from_slice::<Outcome>(&bytes).unwrap_err().to_string();
-        assert!(message.contains("Outcome"), "{label}: {message}");
+        let message = rmp_serde::from_slice::<UnitOutcome>(&bytes).unwrap_err().to_string();
+        assert!(message.contains("UnitOutcome"), "{label}: {message}");
         assert!(!message.contains("CANARY"), "{label}: {message}");
     }
 }
diff --git a/packages/cultcache-ts/test/union-parity.test.ts b/packages/cultcache-ts/test/union-parity.test.ts
index 81cff9c5..15bfaeb9 100644
--- a/packages/cultcache-ts/test/union-parity.test.ts
+++ b/packages/cultcache-ts/test/union-parity.test.ts
@@ -64,7 +64,7 @@ interface Surface {
   id: string;
   version: number;
   root: Node | null;
-  outcomes: (Outcome | null)[];
+  outcomes: (GoldenOutcome | null)[];
   part: Item | null;
   crafted: CraftedItem | null;
   missing: Node | null;
diff --git a/tests/GameCult.Caching.Tests/UnionParityVectorTests.cs b/tests/GameCult.Caching.Tests/UnionParityVectorTests.cs
index 172cf604..237d90e1 100644
--- a/tests/GameCult.Caching.Tests/UnionParityVectorTests.cs
+++ b/tests/GameCult.Caching.Tests/UnionParityVectorTests.cs
@@ -282,7 +282,7 @@ namespace GameCult.Caching.Tests
             new Case("unknown-arm-field.msgpack", "refused", "Node", 3, "unknown-arm",
                 Surface(root: ArmOf(3, Str("r"), Str(Canary)))),
             new Case("unknown-arm-list.msgpack", "refused", "Outcome", 7, "unknown-arm",
-                Surface(outcomes: Slots(ArmOf(0, Str("wait")), ArmOf(7, Str(Canary))))),
+                Surface(outcomes: Slots(ArmOf(0, Str("wait"), Str("0000000000a1")), ArmOf(7, Str(Canary))))),
             new Case("unknown-arm-nested.msgpack", "refused", "Node", 9, "unknown-arm",
                 Surface(root: BoxArm("0000000000c0", Slots(BoxArm("0000000000d3", Slots(BoxArm("0000000000d4", Slots(ArmOf(9, Str("deep"), Str(Canary)))))))))),
             new Case("inner-union-outer-key.msgpack", "refused", "CraftedItem", 2, "unknown-arm",