diff --git a/tests/GameCult.Caching.Tests/UnionArmGuardTests.cs b/tests/GameCult.Caching.Tests/UnionArmGuardTests.cs
index e6a11309..eed82076 100644
--- a/tests/GameCult.Caching.Tests/UnionArmGuardTests.cs
+++ b/tests/GameCult.Caching.Tests/UnionArmGuardTests.cs
@@ -357,43 +357,44 @@ namespace GameCult.Caching.Tests
             Assert.That(Convert.ToHexString(plainBytes), Is.EqualTo(Convert.ToHexString(MessagePackSerializer.Serialize(plain, CultDocumentMessagePackSerialization.Options))));
         }
 
-        // StandardResolverAllowPrivate cannot always build a formatter (a private readonly [Key] field has no constructor to
-        // fill it). That is not an escape from the claim check: a resolver formatting such a type is refused like any other.
+        // StandardResolverAllowPrivate cannot always build a formatter (a private [Key] delegate member has none). That is not an
+        // escape from the claim check: a resolver formatting such a type is refused like any other.
         [Test]
         public void AFormatterThatStandardResolverAllowPrivateCannotBuildStillClaims()
         {
-            var registry = CultDocumentRegistry.ForTypes(new[] { typeof(ApRoDoc) });
-            var doc = new ApRoDoc { Id = "r1", Items = { new ApRoArm { Id = "00000000000c" } } };
+            var registry = CultDocumentRegistry.ForTypes(new[] { typeof(ApHookDoc) });
+            var doc = new ApHookDoc { Id = "r1", Items = { new ApHookArm { Id = "00000000000c" } } };
 
-            var thrown = Assert.Throws<InvalidOperationException>(() => CultDocumentMessagePackSerialization.SerializeUntyped(doc, typeof(ApRoDoc), registry))!;
+            var thrown = Assert.Throws<InvalidOperationException>(() => CultDocumentMessagePackSerialization.SerializeUntyped(doc, typeof(ApHookDoc), registry))!;
 
-            Assert.That(thrown.Message, Does.Contain(nameof(ListClaimResolver)).And.Contain(nameof(ApRoArm)));
+            Assert.That(thrown.Message, Does.Contain(nameof(ListClaimResolver)).And.Contain(nameof(ApHookArm)));
         }
 
         [MessagePackObject]
-        public sealed class ApRoArm
+        public sealed class ApHookArm
         {
             [Key(0)] [CultElementId] public string Id = string.Empty;
-            [Key(1)] private readonly int fixedValue = 3;
-            [IgnoreMember] public int FixedValue => fixedValue;
+#pragma warning disable CS0169, CS0649
+            [Key(1)] private Action? hook;
+#pragma warning restore CS0169, CS0649
         }
 
-        [CultDocument("tests.ap_ro_doc", "tests.ap_ro_doc.v1")]
+        [CultDocument("tests.ap_hook_doc", "tests.ap_hook_doc.v1")]
         [MessagePackObject]
-        public sealed class ApRoDoc
+        public sealed class ApHookDoc
         {
             [Key(0)] [CultName] public string Id = string.Empty;
-            [Key(1)] public List<ApRoArm> Items = new();
+            [Key(1)] public List<ApHookArm> Items = new();
         }
 
-        private sealed class ApRoFormatter : IMessagePackFormatter<ApRoArm>
+        private sealed class ApHookFormatter : IMessagePackFormatter<ApHookArm>
         {
-            public void Serialize(ref MessagePackWriter writer, ApRoArm value, MessagePackSerializerOptions options) => writer.WriteNil();
+            public void Serialize(ref MessagePackWriter writer, ApHookArm value, MessagePackSerializerOptions options) => writer.WriteNil();
 
-            public ApRoArm Deserialize(ref MessagePackReader reader, MessagePackSerializerOptions options)
+            public ApHookArm Deserialize(ref MessagePackReader reader, MessagePackSerializerOptions options)
             {
                 reader.Skip();
-                return new ApRoArm();
+                return new ApHookArm();
             }
         }
 
@@ -497,7 +498,7 @@ namespace GameCult.Caching.Tests
                 if (typeof(T) == typeof(List<LcValue>)) return (IMessagePackFormatter<T>)(object)new ValuesFormatter();
                 if (typeof(T) == typeof(List<Pair>)) return (IMessagePackFormatter<T>)(object)new PointsFormatter();
                 if (typeof(T) == typeof(FbArm)) return global::MessagePack.Resolvers.StandardResolver.Instance.GetFormatter<T>();
-                if (typeof(T) == typeof(ApRoArm)) return (IMessagePackFormatter<T>)(object)new ApRoFormatter();
+                if (typeof(T) == typeof(ApHookArm)) return (IMessagePackFormatter<T>)(object)new ApHookFormatter();
                 if (typeof(T) == typeof(ApArm) || typeof(T) == typeof(ApDoc) || typeof(T) == typeof(List<ApArm>) ||
                     typeof(T) == typeof(ApPlainArm) || typeof(T) == typeof(ApPlainDoc) || typeof(T) == typeof(List<ApPlainArm>))
                     return global::MessagePack.Resolvers.StandardResolverAllowPrivate.Instance.GetFormatter<T>();