|   File "/usr/local/lib/python3.11/site-packages/fastapi/routing.py", line 212, in run_endpoint_function
    |   File "/workspace/bulkmailer/app/api/routers/campaigns.py", line 351, in preview_campaign
    |   File "/workspace/bulkmailer/app/services/message_builder.py", line 115, in sanitize_preview
    | TypeError: unsupported operand type(s) for +: 'frozenset' and 'list'
  File "/usr/local/lib/python3.11/site-packages/fastapi/routing.py", line 212, in run_endpoint_function
  File "/workspace/bulkmailer/app/api/routers/campaigns.py", line 351, in preview_campaign
  File "/workspace/bulkmailer/app/services/message_builder.py", line 115, in sanitize_preview
TypeError: unsupported operand type(s) for +: 'frozenset' and 'list'