`Serial` field won't work in SQLite if it is not the primary key

[metakot]

piccolo==1.5.0
piccolo_admin==1.3.2
piccolo_api==1.4.0

So I have a table in SQLite:

class Example(Table):
    number = Serial()

When trying to save it

example = Example()
await example.save()

the sqlite error occuring:

sqlite3.IntegrityError: NOT NULL constraint failed: example.number

Apparently piccolo creates serial field as just INTEGER without AUTOINCREMENT which is needed by SQLite.