diff --git a/src/Marten/Internal/Sessions/DocumentSessionBase.cs b/src/Marten/Internal/Sessions/DocumentSessionBase.cs
index 536a4e1f99..a3d5c9dc72 100644
--- a/src/Marten/Internal/Sessions/DocumentSessionBase.cs
+++ b/src/Marten/Internal/Sessions/DocumentSessionBase.cs
@@ -399,9 +399,16 @@ public void EjectPatchedTypes(IUnitOfWork changes)
 
     internal void StoreDocumentInItemMap<TDoc, TId>(TId id, TDoc document) where TDoc : class where TId : notnull
     {
-        if (ItemMap.ContainsKey(typeof(TDoc)))
+        if (ItemMap.TryGetValue(typeof(TDoc), out var existing))
         {
-            ItemMap[typeof(TDoc)].As<Dictionary<TId, TDoc>>()[id] = document;
+            if (existing is Dictionary<TId, TDoc> typedDict)
+            {
+                typedDict[id] = document;
+            }
+            // else: The identity map was created with a different key type (e.g., a strong-typed ID
+            // like PaymentId while TId is Guid). The document is already stored by the inline
+            // projection under the strong-typed key, so we skip storing it again to avoid
+            // replacing the dictionary with an incompatible key type.
         }
         else
         {
diff --git a/src/ValueTypeTests/Bugs/Bug_4214_identity_map_strong_typed_ids.cs b/src/ValueTypeTests/Bugs/Bug_4214_identity_map_strong_typed_ids.cs
new file mode 100644
index 0000000000..e1752573f5
--- /dev/null
+++ b/src/ValueTypeTests/Bugs/Bug_4214_identity_map_strong_typed_ids.cs
@@ -0,0 +1,109 @@
+using System;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using System.Threading.Tasks;
+using JasperFx.Events;
+using JasperFx.Events.Projections;
+using Marten;
+using Marten.Events.Aggregation;
+using Marten.Events.Projections;
+using Marten.Testing.Harness;
+using Shouldly;
+using StronglyTypedIds;
+using Xunit;
+
+namespace ValueTypeTests.Bugs;
+
+public class Bug_4214_identity_map_strong_typed_ids : BugIntegrationContext
+{
+    [Theory]
+    [InlineData(ProjectionLifecycle.Inline)]
+    [InlineData(ProjectionLifecycle.Live)]
+    public async Task fetch_for_writing_with_identity_map_and_strong_typed_guid_id(ProjectionLifecycle lifecycle)
+    {
+        StoreOptions(opts =>
+        {
+            opts.UseSystemTextJsonForSerialization(new JsonSerializerOptions { IncludeFields = true });
+            opts.Projections.Add(new SingleStreamProjection<Bug4214Payment, Bug4214PaymentId>(), lifecycle);
+            opts.Events.UseIdentityMapForAggregates = true;
+        });
+
+        await using var session = theStore.LightweightSession();
+
+        var id = session.Events.StartStream<Bug4214Payment>(
+            new Bug4214PaymentCreated(DateTimeOffset.UtcNow),
+            new Bug4214PaymentVerified(DateTimeOffset.UtcNow)).Id;
+
+        await session.SaveChangesAsync();
+
+        // This threw InvalidCastException before the fix:
+        // "Unable to cast object of type 'Dictionary`2[Bug4214PaymentId,Bug4214Payment]'
+        // to type 'Dictionary`2[Guid,Bug4214Payment]'"
+        var stream = await session.Events.FetchForWriting<Bug4214Payment>(id);
+        stream.Aggregate.ShouldNotBeNull();
+        stream.Aggregate.State.ShouldBe(Bug4214PaymentState.Verified);
+    }
+
+    [Theory]
+    [InlineData(ProjectionLifecycle.Inline)]
+    [InlineData(ProjectionLifecycle.Live)]
+    public async Task fetch_for_writing_twice_with_identity_map_and_strong_typed_guid_id(
+        ProjectionLifecycle lifecycle)
+    {
+        StoreOptions(opts =>
+        {
+            opts.UseSystemTextJsonForSerialization(new JsonSerializerOptions { IncludeFields = true });
+            opts.Projections.Add(new SingleStreamProjection<Bug4214Payment, Bug4214PaymentId>(), lifecycle);
+            opts.Events.UseIdentityMapForAggregates = true;
+        });
+
+        await using var session = theStore.LightweightSession();
+
+        var id = session.Events.StartStream<Bug4214Payment>(
+            new Bug4214PaymentCreated(DateTimeOffset.UtcNow),
+            new Bug4214PaymentVerified(DateTimeOffset.UtcNow)).Id;
+
+        await session.SaveChangesAsync();
+
+        // First fetch stores in identity map
+        var stream1 = await session.Events.FetchForWriting<Bug4214Payment>(id);
+        stream1.Aggregate.ShouldNotBeNull();
+
+        stream1.AppendOne(new Bug4214PaymentCanceled(DateTimeOffset.UtcNow));
+        await session.SaveChangesAsync();
+
+        // Second fetch should retrieve from identity map without cast error
+        var stream2 = await session.Events.FetchForWriting<Bug4214Payment>(id);
+        stream2.Aggregate.ShouldNotBeNull();
+        stream2.Aggregate.State.ShouldBe(Bug4214PaymentState.Canceled);
+    }
+}
+
+[StronglyTypedId(Template.Guid)]
+public readonly partial struct Bug4214PaymentId;
+
+public class Bug4214Payment
+{
+    [JsonInclude] public Bug4214PaymentId? Id { get; private set; }
+    [JsonInclude] public DateTimeOffset CreatedAt { get; private set; }
+    [JsonInclude] public Bug4214PaymentState State { get; private set; }
+
+    public static Bug4214Payment Create(IEvent<Bug4214PaymentCreated> @event)
+    {
+        return new Bug4214Payment
+        {
+            Id = new Bug4214PaymentId(@event.StreamId),
+            CreatedAt = @event.Data.CreatedAt,
+            State = Bug4214PaymentState.Created
+        };
+    }
+
+    public void Apply(Bug4214PaymentVerified _) => State = Bug4214PaymentState.Verified;
+    public void Apply(Bug4214PaymentCanceled _) => State = Bug4214PaymentState.Canceled;
+}
+
+public enum Bug4214PaymentState { Created, Verified, Canceled }
+
+public record Bug4214PaymentCreated(DateTimeOffset CreatedAt);
+public record Bug4214PaymentVerified(DateTimeOffset VerifiedAt);
+public record Bug4214PaymentCanceled(DateTimeOffset CanceledAt);