I specified json.loads as the KafkaConsumer's key_deserializer callable, then produced a message without a key.

json.loads throws an exception about expecting a string. Looks like root cause is here: https://github.com/dpkp/kafka-python/blob/master/kafka/consumer/fetcher.py#L546

Fetcher is always using passing the key to the deserializer, even if the key is None.

Will submit a PR shortly.