diff --git a/backend/alembic/versions/78fc200abac1_add_account_id_script_tables.py b/backend/alembic/versions/78fc200abac1_add_account_id_script_tables.py
index 21cf8f2b..74116db6 100644
--- a/backend/alembic/versions/78fc200abac1_add_account_id_script_tables.py
+++ b/backend/alembic/versions/78fc200abac1_add_account_id_script_tables.py
@@ -13,8 +13,24 @@ down_revision: Union[str, None] = '7f136778f5a8'
 branch_labels: Union[str, Sequence[str], None] = None
 depends_on: Union[str, Sequence[str], None] = None
 
+PLATFORM_ACCOUNT_ID = '00000000-0000-0000-0000-000000000001'
+
 
 def upgrade() -> None:
+    # Ensure the platform sentinel account exists before any fallback assignments.
+    # Migration 3a40fe11b427 also inserts this with ON CONFLICT DO NOTHING — safe.
+    op.execute(f"""
+        INSERT INTO accounts (id, name, display_code, created_at, updated_at)
+        VALUES (
+            '{PLATFORM_ACCOUNT_ID}',
+            'ResolutionFlow Platform',
+            'PLATFORM',
+            NOW(),
+            NOW()
+        )
+        ON CONFLICT (id) DO NOTHING
+    """)
+
     for table in ('script_builder_sessions', 'script_templates', 'script_generations'):
         op.add_column(table, sa.Column('account_id', sa.UUID(), nullable=True))
         op.create_foreign_key(
@@ -39,7 +55,7 @@ def upgrade() -> None:
         WHERE st.created_by = u.id
           AND st.account_id IS NULL
     """)
-    # Fallback for script_templates with NULL created_by: team_id → team admin user
+    # Fallback: team_id → team admin user
     op.execute("""
         UPDATE script_templates st
         SET account_id = u.account_id
@@ -49,6 +65,13 @@ def upgrade() -> None:
           AND u.account_id IS NOT NULL
           AND st.account_id IS NULL
     """)
+    # Final fallback: platform-seeded templates with NULL team_id AND NULL created_by
+    # (e.g. the 6 AD templates inserted by migration 057) → platform sentinel account
+    op.execute(f"""
+        UPDATE script_templates
+        SET account_id = '{PLATFORM_ACCOUNT_ID}'
+        WHERE account_id IS NULL
+    """)
 
     # script_generations: user_id → users.account_id
     op.execute("""