Django ModelAdmin - fieldsets ... field 'date' missing from the form

I figured out what the problem was while writing this question. I post it anyway in case it could help someone else.

The error: 'FooAdmin.fieldsets[0][1]['fields']' refers to field 'date' that is missing from the form.

With the following code:

# models.py
from django.db import models

class Foo(Base):
    date = models.DateField(auto_now_add=True)
    title = models.CharField(max_length=255)

# admin.py
from django.contrib import admin

class FooAdmin(BaseAdmin):
    list_display = ("title", "date")
    fieldsets = (
        (None, {
            "fields": ("date", "title")
        }),
    )

admin.site.register(Foo, FooAdmin)