Column unknown tablename.RDB$DB_KEY

[gribok]

I have a legacy database table without a primary key.
Firebird is using RDB$DB_KEY as alternative internal primary key.

I tried to model it in Django.
But when I execute a SELECT statement, I receive following error message:
('Error while preparing SQL statement:\n- SQLCODE: -206\n- Dynamic SQL Error\n- SQL error code = -206\n- Column unknown\n- testtable.RDB$DB_KEY\n- At line 1, column 18', -206, 335544569)

I've tried it with Django 2.2, but got the same error message.

myappmodels.py

from django.db import models

# Create your models here.

class MyModel(models.Model):
    db_key = models.CharField(primary_key=True, db_column='rdb$db_key', max_length=10)
    test_field= models.CharField(max_length=15, blank=True, null=True)

    class Meta:
        managed = False
        db_table = 'testtable'

Version
Django: 3.2
django-firebird: 2.2.1
django-utils-six 2.0
fdb 2.0.2

$ python -V
Python 3.9.15

The SELECT statement with via fdb package works perfectly.