diff --git a/src/Infrastructure.EntityFramework/AdminConsole/Configurations/CollectionEntityTypeConfiguration.cs b/src/Infrastructure.EntityFramework/AdminConsole/Configurations/CollectionEntityTypeConfiguration.cs
index 24c56ac4e1..11975a1b9e 100644
--- a/src/Infrastructure.EntityFramework/AdminConsole/Configurations/CollectionEntityTypeConfiguration.cs
+++ b/src/Infrastructure.EntityFramework/AdminConsole/Configurations/CollectionEntityTypeConfiguration.cs
@@ -18,6 +18,14 @@ public class CollectionEntityTypeConfiguration : IEntityTypeConfiguration<Collec
             .HasFilter("[Type] = 1")
             .HasDatabaseName("IX_Collection_DefaultCollectionOwner_OrganizationId_Type");
 
+        // Configure FK with NO ACTION delete behavior to prevent cascade conflicts
+        // Cleanup is handled explicitly in OrganizationUserRepository.DeleteAsync
+        builder
+            .HasOne<OrganizationUser>()
+            .WithMany()
+            .HasForeignKey(c => c.DefaultCollectionOwner)
+            .OnDelete(DeleteBehavior.NoAction);
+
         builder.ToTable(nameof(Collection));
     }
 }
\ No newline at end of file
diff --git a/src/Infrastructure.EntityFramework/AdminConsole/Repositories/OrganizationUserRepository.cs b/src/Infrastructure.EntityFramework/AdminConsole/Repositories/OrganizationUserRepository.cs
index ae55099775..c8c453f224 100644
--- a/src/Infrastructure.EntityFramework/AdminConsole/Repositories/OrganizationUserRepository.cs
+++ b/src/Infrastructure.EntityFramework/AdminConsole/Repositories/OrganizationUserRepository.cs
@@ -114,7 +114,8 @@ public class OrganizationUserRepository : Repository<Core.Entities.OrganizationU
                         .SetProperty(c => c.Type, CollectionType.SharedCollection)
                         .SetProperty(c => c.RevisionDate, utcNow)
                         .SetProperty(c => c.DefaultUserCollectionEmail,
-                            c => c.DefaultUserCollectionEmail == null ? email : c.DefaultUserCollectionEmail));
+                            c => c.DefaultUserCollectionEmail == null ? email : c.DefaultUserCollectionEmail)
+                        .SetProperty(c => c.DefaultCollectionOwner, (Guid?)null));
 
                 await dbContext.CollectionUsers
                     .Where(cu => cu.OrganizationUserId == organizationUser.Id)
diff --git a/src/Sql/dbo/Stored Procedures/OrganizationUser_MigrateDefaultCollection.sql b/src/Sql/dbo/Stored Procedures/OrganizationUser_MigrateDefaultCollection.sql
index f65cdc3983..16be582485 100644
--- a/src/Sql/dbo/Stored Procedures/OrganizationUser_MigrateDefaultCollection.sql	
+++ b/src/Sql/dbo/Stored Procedures/OrganizationUser_MigrateDefaultCollection.sql	
@@ -9,7 +9,8 @@ BEGIN
     SET
         [DefaultUserCollectionEmail] = CASE WHEN c.[DefaultUserCollectionEmail] IS NULL THEN u.[Email] ELSE c.[DefaultUserCollectionEmail] END,
         [RevisionDate] = @UtcNow,
-        [Type] = 0
+        [Type] = 0,
+        [DefaultCollectionOwner] = NULL
     FROM
         [dbo].[Collection] c
         INNER JOIN [dbo].[CollectionUser] cu ON c.[Id] = cu.[CollectionId]
diff --git a/src/Sql/dbo/Tables/Collection.sql b/src/Sql/dbo/Tables/Collection.sql
index be9a39278e..9b3009d4fe 100644
--- a/src/Sql/dbo/Tables/Collection.sql
+++ b/src/Sql/dbo/Tables/Collection.sql
@@ -10,7 +10,7 @@
     [DefaultCollectionOwner]        UNIQUEIDENTIFIER    NULL,
     CONSTRAINT [PK_Collection] PRIMARY KEY CLUSTERED ([Id] ASC),
     CONSTRAINT [FK_Collection_Organization] FOREIGN KEY ([OrganizationId]) REFERENCES [dbo].[Organization] ([Id]) ON DELETE CASCADE,
-    CONSTRAINT [FK_Collection_OrganizationUser] FOREIGN KEY ([DefaultCollectionOwner]) REFERENCES [dbo].[OrganizationUser] ([Id]) ON DELETE SET NULL
+    CONSTRAINT [FK_Collection_OrganizationUser] FOREIGN KEY ([DefaultCollectionOwner]) REFERENCES [dbo].[OrganizationUser] ([Id]) ON DELETE NO ACTION
 );
 GO
 
diff --git a/test/Infrastructure.IntegrationTest/AdminConsole/Repositories/OrganizationUserRepository/OrganizationUserRepositoryTests.cs b/test/Infrastructure.IntegrationTest/AdminConsole/Repositories/OrganizationUserRepository/OrganizationUserRepositoryTests.cs
index 68d1b29c68..717eafc430 100644
--- a/test/Infrastructure.IntegrationTest/AdminConsole/Repositories/OrganizationUserRepository/OrganizationUserRepositoryTests.cs
+++ b/test/Infrastructure.IntegrationTest/AdminConsole/Repositories/OrganizationUserRepository/OrganizationUserRepositoryTests.cs
@@ -147,11 +147,13 @@ public class OrganizationUserRepositoryTests
         Assert.NotNull(updatedCollection1);
         Assert.Equal(CollectionType.SharedCollection, updatedCollection1.Type);
         Assert.Equal(user1.Email, updatedCollection1.DefaultUserCollectionEmail);
+        Assert.Null(updatedCollection1.DefaultCollectionOwner);
 
         var updatedCollection2 = await collectionRepository.GetByIdAsync(defaultUserCollection2.Id);
         Assert.NotNull(updatedCollection2);
         Assert.Equal(CollectionType.SharedCollection, updatedCollection2.Type);
         Assert.Equal(user2.Email, updatedCollection2.DefaultUserCollectionEmail);
+        Assert.Null(updatedCollection2.DefaultCollectionOwner);
     }
 
     [DatabaseTheory, DatabaseData]
@@ -214,6 +216,7 @@ public class OrganizationUserRepositoryTests
         Assert.NotNull(updatedCollection);
         Assert.Equal(CollectionType.SharedCollection, updatedCollection.Type);
         Assert.Equal(user.Email, updatedCollection.DefaultUserCollectionEmail);
+        Assert.Null(updatedCollection.DefaultCollectionOwner);
     }
 
 
@@ -1355,6 +1358,7 @@ public class OrganizationUserRepositoryTests
         Assert.NotNull(updatedCollection);
         Assert.Equal(CollectionType.SharedCollection, updatedCollection.Type);
         Assert.Equal(user.Email, updatedCollection.DefaultUserCollectionEmail);
+        Assert.Null(updatedCollection.DefaultCollectionOwner);
     }
 
     [DatabaseTheory, DatabaseData]
@@ -1412,6 +1416,7 @@ public class OrganizationUserRepositoryTests
         Assert.NotNull(updatedCollection);
         Assert.Equal(CollectionType.SharedCollection, updatedCollection.Type);
         Assert.Equal(user.Email, updatedCollection.DefaultUserCollectionEmail);
+        Assert.Null(updatedCollection.DefaultCollectionOwner);
     }
 
     [DatabaseTheory, DatabaseData]
diff --git a/util/Migrator/DbScripts/2025-12-20_00_Collection_AddDefaultCollectionOwnerAndConstraint.sql b/util/Migrator/DbScripts/2025-12-20_00_Collection_AddDefaultCollectionOwnerAndConstraint.sql
index 5144d0515f..476c91b56e 100644
--- a/util/Migrator/DbScripts/2025-12-20_00_Collection_AddDefaultCollectionOwnerAndConstraint.sql
+++ b/util/Migrator/DbScripts/2025-12-20_00_Collection_AddDefaultCollectionOwnerAndConstraint.sql
@@ -26,7 +26,7 @@ BEGIN
         ADD CONSTRAINT [FK_Collection_OrganizationUser]
         FOREIGN KEY ([DefaultCollectionOwner])
         REFERENCES [dbo].[OrganizationUser] ([Id])
-        ON DELETE SET NULL
+        ON DELETE NO ACTION
 END
 GO
 
diff --git a/util/Migrator/DbScripts/2025-12-20_02_OrganizationUser_MigrateDefaultCollection.sql b/util/Migrator/DbScripts/2025-12-20_02_OrganizationUser_MigrateDefaultCollection.sql
new file mode 100644
index 0000000000..4f90b6d251
--- /dev/null
+++ b/util/Migrator/DbScripts/2025-12-20_02_OrganizationUser_MigrateDefaultCollection.sql
@@ -0,0 +1,26 @@
+-- Update OrganizationUser_MigrateDefaultCollection to set DefaultCollectionOwner = NULL
+-- when migrating default collections to shared collections during OrganizationUser deletion
+
+CREATE OR ALTER PROCEDURE [dbo].[OrganizationUser_MigrateDefaultCollection]
+    @Ids [dbo].[GuidIdArray] READONLY
+AS
+BEGIN
+    SET NOCOUNT ON
+    DECLARE @UtcNow DATETIME2(7) = GETUTCDATE();
+
+    UPDATE c
+    SET
+        [DefaultUserCollectionEmail] = CASE WHEN c.[DefaultUserCollectionEmail] IS NULL THEN u.[Email] ELSE c.[DefaultUserCollectionEmail] END,
+        [RevisionDate] = @UtcNow,
+        [Type] = 0,
+        [DefaultCollectionOwner] = NULL
+    FROM
+        [dbo].[Collection] c
+        INNER JOIN [dbo].[CollectionUser] cu ON c.[Id] = cu.[CollectionId]
+        INNER JOIN [dbo].[OrganizationUser] ou ON cu.[OrganizationUserId] = ou.[Id]
+        INNER JOIN [dbo].[User] u ON ou.[UserId] = u.[Id]
+        INNER JOIN @Ids i ON ou.[Id] = i.[Id]
+    WHERE
+        c.[Type] = 1
+END
+GO
\ No newline at end of file