ValueError: Expected dict_keys(['answer']) but got dict_keys([])

[rangehow]

I run the official code example in intro.ipynb：

import dspy

lm = dspy.LM(model='openai/default', api_key=" ", api_base=" ",temperature=0.9, max_tokens=3000,)
colbertv2_wiki17_abstracts = dspy.ColBERTv2(url='http://20.102.90.50:2017/wiki17_abstracts')

dspy.settings.configure(lm=lm, rm=colbertv2_wiki17_abstracts)

from dspy.datasets import HotPotQA

# Load the dataset.
dataset = HotPotQA(train_seed=1, train_size=20, eval_seed=2023, dev_size=50, test_size=0)

# Tell DSPy that the 'question' field is the input. Any other fields are labels and/or metadata.
trainset = [x.with_inputs('question') for x in dataset.train]
devset = [x.with_inputs('question') for x in dataset.dev]

len(trainset), len(devset)

train_example = trainset[0]
print(f"Question: {train_example.question}")
print(f"Answer: {train_example.answer}")

dev_example = devset[18]
print(f"Question: {dev_example.question}")
print(f"Answer: {dev_example.answer}")
print(f"Relevant Wikipedia Titles: {dev_example.gold_titles}")


class BasicQA(dspy.Signature):
    """Answer questions with short factoid answers."""

    question = dspy.InputField()
    answer = dspy.OutputField(desc="often between 1 and 5 words")
    
# Define the predictor.
generate_answer = dspy.Predict(BasicQA)

# Call the predictor on a particular input.
pred = generate_answer(question=dev_example.question)

# Print the input and the prediction.
print(f"Question: {dev_example.question}")
print(f"Predicted Answer: {pred.answer}")

ERROR I get

Traceback (most recent call last):
  File "/mnt/rangehow/dspy/1.py", line 39, in <module>
    pred = generate_answer(question=dev_example.question)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/mnt/rangehow/dspy/dspy/predict/predict.py", line 98, in __call__
    return self.forward(**kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^
  File "/mnt/rangehow/dspy/dspy/predict/predict.py", line 131, in forward
    completions = v2_5_generate(lm, config, signature, demos, kwargs)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/mnt/rangehow/dspy/dspy/predict/predict.py", line 231, in v2_5_generate
    return adapter(lm, lm_kwargs=lm_kwargs, signature=signature, demos=demos, inputs=inputs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/mnt/rangehow/dspy/dspy/adapters/base.py", line 11, in __call__
    value = self.parse(signature, output)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/mnt/rangehow/dspy/dspy/adapters/chat_adapter.py", line 58, in parse
    raise ValueError(f"Expected {signature.output_fields.keys()} but got {fields.keys()}")
ValueError: Expected dict_keys(['answer']) but got dict_keys([])

The outputs from LLM, I print it out:

['answer: American\n[[ ## completed ## ]]']