Internal error encountered when using VertexAI in ConversationalRetrievalChain

```
~\Anaconda3\lib\site-packages\langchain\chat_models\base.py in generate_prompt(self, prompts, stop, callbacks)
    141     ) -> LLMResult:
    142         prompt_messages = [p.to_messages() for p in prompts]
--> 143         return self.generate(prompt_messages, stop=stop, callbacks=callbacks)
    144 
    145     async def agenerate_prompt(

~\Anaconda3\lib\site-packages\langchain\chat_models\base.py in generate(self, messages, stop, callbacks)
     89         except (KeyboardInterrupt, Exception) as e:
     90             run_manager.on_llm_error(e)
---> 91             raise e
     92         llm_output = self._combine_llm_outputs([res.llm_output for res in results])
     93         generations = [res.generations for res in results]

~\Anaconda3\lib\site-packages\langchain\chat_models\base.py in generate(self, messages, stop, callbacks)
     81         )
     82         try:
---> 83             results = [
     84                 self._generate(m, stop=stop, run_manager=run_manager)
     85                 if new_arg_supported

~\Anaconda3\lib\site-packages\langchain\chat_models\base.py in <listcomp>(.0)
     82         try:
     83             results = [
---> 84                 self._generate(m, stop=stop, run_manager=run_manager)
     85                 if new_arg_supported
     86                 else self._generate(m, stop=stop)

~\Anaconda3\lib\site-packages\langchain\chat_models\vertexai.py in _generate(self, messages, stop, run_manager)
    123         for pair in history.history:
    124             chat._history.append((pair.question.content, pair.answer.content))
--> 125         response = chat.send_message(question.content)
    126         text = self._enforce_stop_words(response.text, stop)
    127         return ChatResult(generations=[ChatGeneration(message=AIMessage(content=text))])

~\Anaconda3\lib\site-packages\vertexai\language_models\_language_models.py in send_message(self, message, max_output_tokens, temperature, top_k, top_p)
    676             ]
    677 
--> 678         prediction_response = self._model._endpoint.predict(
    679             instances=[prediction_instance],
    680             parameters=prediction_parameters,

~\Anaconda3\lib\site-packages\google\cloud\aiplatform\models.py in predict(self, instances, parameters, timeout, use_raw_predict)
   1544             )
   1545         else:
-> 1546             prediction_response = self._prediction_client.predict(
   1547                 endpoint=self._gca_resource.name,
   1548                 instances=instances,

~\Anaconda3\lib\site-packages\google\cloud\aiplatform_v1\services\prediction_service\client.py in predict(self, request, endpoint, instances, parameters, retry, timeout, metadata)
    600 
    601         # Send the request.
--> 602         response = rpc(
    603             request,
    604             retry=retry,

~\Anaconda3\lib\site-packages\google\api_core\gapic_v1\method.py in __call__(self, timeout, retry, *args, **kwargs)
    111             kwargs["metadata"] = metadata
    112 
--> 113         return wrapped_func(*args, **kwargs)
    114 
    115 

~\Anaconda3\lib\site-packages\google\api_core\grpc_helpers.py in error_remapped_callable(*args, **kwargs)
     72             return callable_(*args, **kwargs)
     73         except grpc.RpcError as exc:
---> 74             raise exceptions.from_grpc_error(exc) from exc
     75 
     76     return error_remapped_callable

InternalServerError: 500 Internal error encountered.
```