validation error for ConversationalRetrievalChain -> combine_docs_chain -> field required (type=value_error.missing)

[umair313]

System: MacOS dependecies:

langchain==0.0.221
openai==0.27.8
tiktoken==0.4.0
jq==1.4.1

code

....
 self.llm = ChatOpenAI(model_name='gpt-3.5-turbo-16k', verbose=True)
 self.llm_chain = LLMChain(
            llm=self.llm,
            prompt=prompt,
        )
 doc_chain = load_qa_chain(self.llm, chain_type="map_reduce")
 self.conversational_chain = ConversationalRetrievalChain(
            retriever=self.retriever,
            question_generator=self.llm_chain,
            combine_docs_chain=doc_chain,
        )
.....