diff --git a/backend/alembic/versions/030_add_user_id_to_calendars.py b/backend/alembic/versions/030_add_user_id_to_calendars.py
index ef1b621..23aafe1 100644
--- a/backend/alembic/versions/030_add_user_id_to_calendars.py
+++ b/backend/alembic/versions/030_add_user_id_to_calendars.py
@@ -15,11 +15,15 @@ depends_on = None
 
 def upgrade() -> None:
     op.add_column("calendars", sa.Column("user_id", sa.Integer(), nullable=True))
+    # Backfill existing calendars to first admin user
     op.execute(
         "UPDATE calendars SET user_id = ("
         "  SELECT id FROM users WHERE role = 'admin' ORDER BY id LIMIT 1"
         ")"
     )
+    # On fresh installs no users exist yet, so seeded calendars still have
+    # NULL user_id.  Remove them — account setup will recreate defaults.
+    op.execute("DELETE FROM calendars WHERE user_id IS NULL")
     op.create_foreign_key(
         "fk_calendars_user_id", "calendars", "users",
         ["user_id"], ["id"], ondelete="CASCADE"