Django addConstraints raises a TypeError on Postgres

I have in models.py a model called FlashNews, that can either be tied to a Race, a League, or a FantasyTeam. I used Django's CheckConstraint (version 2.2.1). There's also a type column to ensure consistency.

I want to guarantee that one and only one column is not null among race, fteam and league, consistently with type value.

The sample code is as is:

FlashNewsTypes = [
    (1, 'Race'),
    (2, 'League'),
    (3, 'Fteam')
]


class FlashNews(models.Model):

    race = models.ForeignKey(Race, on_delete=models.CASCADE, null=True, blank=True)
    league = models.ForeignKey(League, on_delete=models.CASCADE, null=True, blank=True)
    fteam = models.ForeignKey(FantasyTeam, on_delete=models.CASCADE, null=True, blank=True)
    type = models.IntegerField(choices=FlashNewsTypes, default=FlashNewsTypes[0][0])

    class Meta:
        constraints = [
            models.CheckConstraint(
                name="unique_notnull_field",
                check=(
                    models.Q(
                        type=FlashNewsTypes[0][0],
                        race__isnull=False,
                        league__isnull=True,
                        fteam__isnull=True,
                    ) | models.Q(
                        type=FlashNewsTypes[1][0],
                        race__isnull=True,
                        league__isnull=False,
                        fteam__isnull=True,
                    ) | models.Q(
                        type=FlashNewsTypes[2][0],
                        race__isnull=True,
                        league__isnull=True,
                        fteam__isnull=False,
                    )
                ),
            )
        ]

The automatically created migration:

class Migration(migrations.Migration):

    dependencies = [
        ('myapp', '0055_flashnews'),
    ]

    operations = [
        migrations.AddConstraint(
            model_name='flashnews',
            constraint=models.CheckConstraint(check=models.Q(models.Q(('fteam__isnull', True), ('league__isnull', True), ('race__isnull', False), ('type', 1)), models.Q(('fteam__isnull', False), ('league__isnull', True), ('race__isnull', True), ('type', 2)), models.Q(('fteam__isnull', True), ('league__isnull', False), ('race__isnull', True), ('type', 3)), _connector='OR'), name='%(app_label)s_%(class)s_value_matches_type'),
        ),
    ]

This migration is successful on my dev env, which uses Sqlite backend. But, when executed with Postgres backend, it raises this error:

Applying myapp.0056_auto_20200502_1754...Traceback (most recent call last):
  File "manage.py", line 22, in <module>
    execute_from_command_line(sys.argv)
  File "/home/src/myapp/env/lib/python3.6/site-packages/django/core/management/__init__.py", line 381, in execute_from_command_line
    utility.execute()
  File "/home/src/myapp/env/lib/python3.6/site-packages/django/core/management/__init__.py", line 375, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/src/myapp/env/lib/python3.6/site-packages/django/core/management/base.py", line 323, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/home/src/myapp/env/lib/python3.6/site-packages/django/core/management/base.py", line 364, in execute
    output = self.handle(*args, **options)
  File "/home/src/myapp/env/lib/python3.6/site-packages/django/core/management/base.py", line 83, in wrapped
    res = handle_func(*args, **kwargs)
  File "/home/src/myapp/env/lib/python3.6/site-packages/django/core/management/commands/migrate.py", line 234, in handle
    fake_initial=fake_initial,
  File "/home/src/myapp/env/lib/python3.6/site-packages/django/db/migrations/executor.py", line 117, in migrate
    state = self._migrate_all_forwards(state, plan, full_plan, fake=fake, fake_initial=fake_initial)
  File "/home/src/myapp/env/lib/python3.6/site-packages/django/db/migrations/executor.py", line 147, in _migrate_all_forwards
    state = self.apply_migration(state, migration, fake=fake, fake_initial=fake_initial)
  File "/home/src/myapp/env/lib/python3.6/site-packages/django/db/migrations/executor.py", line 245, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/home/src/myapp/env/lib/python3.6/site-packages/django/db/migrations/migration.py", line 124, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "/home/src/myapp/env/lib/python3.6/site-packages/django/db/migrations/operations/models.py", line 827, in database_forwards
    schema_editor.add_constraint(model, self.constraint)
  File "/home/src/myapp/env/lib/python3.6/site-packages/django/db/backends/base/schema.py", line 345, in add_constraint
    self.execute(sql)
  File "/home/src/myapp/env/lib/python3.6/site-packages/django/db/backends/base/schema.py", line 137, in execute
    cursor.execute(sql, params)
  File "/home/src/myapp/env/lib/python3.6/site-packages/django/db/backends/utils.py", line 67, in execute
    return self._execute_with_wrappers(sql, params, many=False, executor=self._execute)
  File "/home/src/myapp/env/lib/python3.6/site-packages/django/db/backends/utils.py", line 76, in _execute_with_wrappers
    return executor(sql, params, many, context)
  File "/home/src/myapp/env/lib/python3.6/site-packages/django/db/backends/utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)
TypeError: tuple indices must be integers or slices, not str

Are there some DB backend-specific notions I'm missing here regarding Django's constraint ?

Thanks for your help !