During install, syncdb fails

[alex-protean]

I'm following the install process in the docs, and the third step, syncdb, fails. This is the issue, but I'm not sure how to fix it:

File ".virtualenvs/test/lib/python3.6/site-packages/django_yubico/models.py", line 5, in <module>
    class YubicoKey(models.Model):
  File ".virtualenvs/test/lib/python3.6/site-packages/django_yubico/models.py", line 9, in YubicoKey
    user = models.ForeignKey(User)
TypeError: __init__() missing 1 required positional argument: 'on_delete'

I am using an extended user model, subclassing abstractuser. Could that have something to do with it?

[Frikster]

This library only supports Django 1.0 - 1.5. Django 2.0+ made on_delete mandatory. I'm assuming you were using a later version of Django since the earlier ones have security issues today.