dumpdata fails with crytpic error

Just tried this:

$ python3 manage.py dumpdata --format json --indent 4 > data.json
CommandError: Unable to serialize database: 'NoneType' object has no attribute 'is_authenticated'

Anyone seen this before. How does one diagnose this given the vague nature of the message? I mean I presume it's complaining because there's an illegal value in a field in the database. But you think it's mention which field in which model on which record? Is there a quick way to find out?

Regards,

Bernd.

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