Getting KeyError: {'context', 'question'} when following Chat Vector DB with streaming example
I'm trying to follow this example [https://langchain.readthedocs.io/en/latest/modules/indexes/chain_examples/chat_vector_db.html?highlight=chatvectordb#chat-vector-db-with-streaming-to-stdout](url)
and I've used PagedPDFSplitter to load a PDF.
This is how I've done it (build_vectorstore returns Chroma.from_documents(texts,embeddings))
```
documents = load_documents()
texts= split_text(documents)
vectorstore = build_vectorstore(texts)

llm = ChatOpenAI(temperature=0)
streaming_llm = ChatOpenAI(streaming=True,callback_manager=CallbackManager([StreamingStdOutCallbackHandler()]),verbose=True,temperature=0)
question_generator = LLMChain(llm=llm,prompt=CONDENSE_QUESTION_PROMPT)
doc_chain = load_qa_chain(streaming_llm,chain_type="stuff",prompt=QA_PROMPT)

qa = ChatVectorDBChain(vectorstore=vectorstore,combine_docs_chain=doc_chain,question_generator=question_generator)

chat_history = []
query = "What does this document contain?"
result = qa({"question": query, "chat_history": chat_history})
```
And this is the error I'm getting:

KeyError: {'context', 'question'}

```
[91] result = qa({"question": query, "chat_history": chat_history})
[114](file:///c%3A/Users//miniconda3/envs/langchain2/lib/site-packages/langchain/chains/base.py?line=113) except (KeyboardInterrupt, Exception) as e:
    [115](file:///c%3A/Users//miniconda3/envs/langchain2/lib/site-packages/langchain/chains/base.py?line=114)     self.callback_manager.on_chain_error(e, verbose=self.verbose)
--> [116](file:///c%3A/Users//miniconda3/envs/langchain2/lib/site-packages/langchain/chains/base.py?line=115)     raise e
    [117](file:///c%3A/Users//miniconda3/envs/langchain2/lib/site-packages/langchain/chains/base.py?line=116) self.callback_manager.on_chain_end(outputs, verbose=self.verbose)
    [118](file:///c%3A/Users//miniconda3/envs/langchain2/lib/site-packages/langchain/chains/base.py?line=117) return self.prep_outputs(inputs, outputs, return_only_outputs)
[107](file:///c%3A/Users//miniconda3/envs/langchain2/lib/site-packages/langchain/chains/base.py?line=106) self.callback_manager.on_chain_start(
    [108](file:///c%3A/Users//miniconda3/envs/langchain2/lib/site-packages/langchain/chains/base.py?line=107)     {"name": self.__class__.__name__},
    [109](file:///c%3A/Users//miniconda3/envs/langchain2/lib/site-packages/langchain/chains/base.py?line=108)     inputs,
    [110](file:///c%3A/Users//miniconda3/envs/langchain2/lib/site-packages/langchain/chains/base.py?line=109)     verbose=self.verbose,
    [111](file:///c%3A/Users//miniconda3/envs/langchain2/lib/site-packages/langchain/chains/base.py?line=110) )
    [112](file:///c%3A/Users//miniconda3/envs/langchain2/lib/site-packages/langchain/chains/base.py?line=111) try:
--> [113](file:///c%3A/Users//miniconda3/envs/langchain2/lib/site-packages/langchain/chains/base.py?line=112)     outputs = self._call(inputs)
    [114](file:///c%3A/Users//miniconda3/envs/langchain2/lib/site-packages/langchain/chains/base.py?line=113) except (KeyboardInterrupt, Exception) as e:
    [115](file:///c%3A/Users//miniconda3/envs/langchain2/lib/site-packages/langchain/chains/base.py?line=114)     self.callback_manager.on_chain_error(e, verbose=self.verbose)
...
     [16](file:///c%3A/Users//miniconda3/envs/langchain2/lib/site-packages/langchain/formatting.py?line=15) extra = set(kwargs).difference(used_args)
     [17](file:///c%3A/Users//miniconda3/envs/langchain2/lib/site-packages/langchain/formatting.py?line=16) if extra:
---> [18](file:///c%3A/Users//miniconda3/envs/langchain2/lib/site-packages/langchain/formatting.py?line=17)     raise KeyError(extra)

KeyError: {'context', 'question'}
```