diff --git a/SVSim.EmulatedEntrypoint/Controllers/UserMyPageController.cs b/SVSim.EmulatedEntrypoint/Controllers/UserMyPageController.cs
index 4e942c0..33eff81 100644
--- a/SVSim.EmulatedEntrypoint/Controllers/UserMyPageController.cs
+++ b/SVSim.EmulatedEntrypoint/Controllers/UserMyPageController.cs
@@ -32,6 +32,8 @@ public sealed class UserMyPageController : SVSimController
         viewer.MyPageBgSelectType = request.SelectType;
         viewer.MyPageBgId = ParseIdOrZero(request.MyPageId);
 
+        // Clear() on a loaded OwnsMany marks every tracked entry as Deleted; SaveChangesAsync
+        // issues DELETEs for all old slots before inserting the new ones.
         viewer.MyPageBgRotation.Clear();
         for (int slot = 0; slot < request.MyPageIdList.Count; slot++)
         {
diff --git a/SVSim.UnitTests/Controllers/UserMyPageControllerTests.cs b/SVSim.UnitTests/Controllers/UserMyPageControllerTests.cs
index fb20901..bb26b2c 100644
--- a/SVSim.UnitTests/Controllers/UserMyPageControllerTests.cs
+++ b/SVSim.UnitTests/Controllers/UserMyPageControllerTests.cs
@@ -146,4 +146,23 @@ public class UserMyPageControllerTests
         // controller's literal return value is what comes back. An empty class serializes to "{}".
         Assert.That(raw, Is.EqualTo("{}"));
     }
+
+    [Test]
+    public async Task Update_with_unparseable_mypage_id_list_entries_falls_back_to_zero()
+    {
+        using var factory = new SVSimTestFactory();
+        long viewerId = await factory.SeedViewerAsync();
+        using var client = factory.CreateAuthenticatedClient(viewerId);
+
+        var body = JsonBody("""
+            {"select_type":2,"mypage_id":"0","mypage_id_list":["1001","garbage","","1002"]}
+            """);
+        var response = await client.PostAsync("/user_mypage/update", body);
+        Assert.That(response.StatusCode, Is.EqualTo(HttpStatusCode.OK));
+
+        var viewer = await LoadViewerWithRotation(factory, viewerId);
+        var pool = viewer.MyPageBgRotation.OrderBy(r => r.Slot).Select(r => r.BgId).ToList();
+        Assert.That(pool, Is.EqualTo(new[] { 1001, 0, 0, 1002 }),
+            "garbage and empty entries are stored as 0; valid entries unaffected");
+    }
 }