diff --git a/src/Persistence/PolecatTests/Bugs/Bug_3942_nullable_aggregate_id_type.cs b/src/Persistence/PolecatTests/Bugs/Bug_3942_nullable_aggregate_id_type.cs
new file mode 100644
index 0000000000..6dfa6024fa
--- /dev/null
+++ b/src/Persistence/PolecatTests/Bugs/Bug_3942_nullable_aggregate_id_type.cs
@@ -0,0 +1,121 @@
+using Shouldly;
+using Wolverine.Polecat.Persistence.Sagas;
+using Wolverine.Polecat.Requirements;
+
+namespace PolecatTests.Bugs;
+
+/// <summary>
+///     GH-3942: the two stores have to agree on an aggregate's id type.
+/// </summary>
+/// <remarks>
+///     <c>WriteModelAttribute.Modify</c> asks the provider for the aggregate's id type and
+///     <c>FindIdentity</c> branches on whether the answer is primitive. Marten answers with the
+///     configured document id type, which is never nullable; Polecat reflected the <c>Id</c>
+///     property verbatim, so <c>public AlertId? Id { get; set; }</c> answered
+///     <c>Nullable&lt;AlertId&gt;</c>. That is not primitive, so the <c>IdentifiedBy&lt;T&gt;</c>
+///     escape hatch was skipped and the message was scanned for a <c>Nullable&lt;AlertId&gt;</c>
+///     member — which exists in no codebase. Every such chain failed to build under Polecat while
+///     the identical shared source worked under Marten.
+///     <para>
+///     These are deliberately reflection-level rather than integration tests: the failure is at
+///     codegen time, and under <c>TypeLoadMode.Dynamic</c> a chain only fails when its message first
+///     arrives — so an integration test would have to dispatch every message type to see it.
+///     </para>
+/// </remarks>
+public class Bug_3942_nullable_aggregate_id_type
+{
+    private static Type sagaIdType<T>() =>
+        new PolecatPersistenceFrameProvider().DetermineSagaIdType(typeof(T), null!);
+
+    // ===== The reported case =====
+
+    [Fact]
+    public void nullable_strong_typed_id_is_unwrapped()
+    {
+        sagaIdType<NullableStrongTypedIdAggregate>().ShouldBe(typeof(AlertId));
+    }
+
+    [Fact]
+    public void nullable_primitive_id_is_unwrapped()
+    {
+        sagaIdType<NullableGuidAggregate>().ShouldBe(typeof(Guid));
+    }
+
+    // ===== Everything else is untouched =====
+
+    [Fact]
+    public void non_nullable_strong_typed_id_is_unchanged()
+    {
+        sagaIdType<StrongTypedIdAggregate>().ShouldBe(typeof(AlertId));
+    }
+
+    [Fact]
+    public void non_nullable_primitive_id_is_unchanged()
+    {
+        sagaIdType<GuidAggregate>().ShouldBe(typeof(Guid));
+    }
+
+    // A string id is already a reference type, so it was never wrapped -- and `string?` erases to
+    // string at runtime, which is exactly why the bug only bit value-type ids.
+    [Fact]
+    public void string_id_is_unchanged()
+    {
+        sagaIdType<StringAggregate>().ShouldBe(typeof(string));
+    }
+
+    [Fact]
+    public void a_type_with_no_id_property_still_falls_back_to_guid()
+    {
+        sagaIdType<NoIdAggregate>().ShouldBe(typeof(Guid));
+    }
+
+    // ===== DocumentExistsAttribute.ResolveIdType is kept in lockstep =====
+
+    [Fact]
+    public void document_id_resolution_agrees_with_the_saga_provider()
+    {
+        DocumentExistsAttribute<NullableStrongTypedIdAggregate>
+            .ResolveIdType(typeof(NullableStrongTypedIdAggregate)).ShouldBe(typeof(AlertId));
+
+        DocumentExistsAttribute<NullableGuidAggregate>
+            .ResolveIdType(typeof(NullableGuidAggregate)).ShouldBe(typeof(Guid));
+
+        DocumentExistsAttribute<GuidAggregate>
+            .ResolveIdType(typeof(GuidAggregate)).ShouldBe(typeof(Guid));
+
+        DocumentExistsAttribute<NoIdAggregate>
+            .ResolveIdType(typeof(NoIdAggregate)).ShouldBe(typeof(Guid));
+    }
+}
+
+public readonly record struct AlertId(string Value);
+
+public class NullableStrongTypedIdAggregate
+{
+    public AlertId? Id { get; set; }
+}
+
+public class StrongTypedIdAggregate
+{
+    public AlertId Id { get; set; }
+}
+
+public class NullableGuidAggregate
+{
+    public Guid? Id { get; set; }
+}
+
+public class GuidAggregate
+{
+    public Guid Id { get; set; }
+}
+
+public class StringAggregate
+{
+    public string Id { get; set; } = string.Empty;
+}
+
+public class NoIdAggregate
+{
+    public string Name { get; set; } = string.Empty;
+}
diff --git a/src/Persistence/Wolverine.Polecat/Persistence/Sagas/PolecatPersistenceFrameProvider.cs b/src/Persistence/Wolverine.Polecat/Persistence/Sagas/PolecatPersistenceFrameProvider.cs
index 6e8202c042..e77535bbc1 100644
--- a/src/Persistence/Wolverine.Polecat/Persistence/Sagas/PolecatPersistenceFrameProvider.cs
+++ b/src/Persistence/Wolverine.Polecat/Persistence/Sagas/PolecatPersistenceFrameProvider.cs
@@ -33,7 +33,16 @@ public bool CanPersist(Type entityType, IServiceContainer container, out Type pe
     public Type DetermineSagaIdType(Type sagaType, IServiceContainer container)
     {
         var idProp = sagaType.GetProperty("Id", BindingFlags.Public | BindingFlags.Instance);
-        return idProp?.PropertyType ?? typeof(Guid);
+        if (idProp == null) return typeof(Guid);
+
+        // GH-3942: unwrap Nullable<T>, so the two stores agree on an aggregate's id type.
+        // Marten answers with the *configured document id type*, which is never nullable. Reflecting
+        // the property verbatim answered Nullable<AlertId> for `public AlertId? Id { get; set; }`,
+        // and WriteModelAttribute.FindIdentity treats that as non-primitive -- so it skipped the
+        // IdentifiedBy<T> escape hatch entirely and went looking for a Nullable<AlertId> member on
+        // the message, which exists nowhere. Every such chain failed to build under Polecat while
+        // the identical source worked under Marten.
+        return Nullable.GetUnderlyingType(idProp.PropertyType) ?? idProp.PropertyType;
     }
 
     public void ApplyTransactionSupport(IChain chain, IServiceContainer container)
diff --git a/src/Persistence/Wolverine.Polecat/Requirements/DocumentExistsAttribute.cs b/src/Persistence/Wolverine.Polecat/Requirements/DocumentExistsAttribute.cs
index 2d51f22b23..f1c4547bdf 100644
--- a/src/Persistence/Wolverine.Polecat/Requirements/DocumentExistsAttribute.cs
+++ b/src/Persistence/Wolverine.Polecat/Requirements/DocumentExistsAttribute.cs
@@ -69,13 +69,18 @@ public override void Modify(IChain chain, GenerationRules rules, IServiceContain
 
     /// <summary>
     /// Resolve the identity type for a Polecat document type by reflecting on the public
-    /// instance <c>Id</c> property. Falls back to <see cref="Guid"/> when the property is
-    /// missing — matching <see cref="Wolverine.Polecat.Persistence.Sagas.PolecatPersistenceFrameProvider.DetermineSagaIdType"/>.
+    /// instance <c>Id</c> property. A nullable id property yields its underlying type, and a
+    /// missing one falls back to <see cref="Guid"/> — matching
+    /// <see cref="Wolverine.Polecat.Persistence.Sagas.PolecatPersistenceFrameProvider.DetermineSagaIdType"/>.
     /// </summary>
     internal static Type ResolveIdType(Type docType)
     {
         var idProp = docType.GetProperty("Id", BindingFlags.Public | BindingFlags.Instance);
-        return idProp?.PropertyType ?? typeof(Guid);
+        if (idProp == null) return typeof(Guid);
+
+        // GH-3942: kept in lockstep with DetermineSagaIdType. See the note there for why the
+        // verbatim property type is wrong for `public MyId? Id { get; set; }`.
+        return Nullable.GetUnderlyingType(idProp.PropertyType) ?? idProp.PropertyType;
     }
 
     internal static bool TryFindIdentityVariable(IChain chain, string? argumentName, Type docType, Type idType,