title: FastAPI: output should be list of lists, but it gives and error about dict... 'value is not a valid dict'
body: <p>I looked at other posts that seems to have similar errors, but they don't seem to be specific to list of lists output. Here is the code:</p>
  <pre><code>@app.post('path/to/file.py', response_model = List[list_data])
  async def function_list_data(something):
      output = data.function_list_data(a = 1, b = 2)
      return output
  
  #&gt; Error: response -&gt; #
      value is not a valid dict (type=type_error.dict)
  </code></pre>
  <p>I've tried many variations and suggestions from other posts. I also tried <code>response_model = List[List[list_data]]</code> but no luck. I don't understand why it's giving me an error about dict.</p>