Why does Django create an index for ForeignKey with db_constraint=False

I want to shard my database so I can't use foreign key constraints. I have following models

from django.db import models


class A(models.Model):
    b_one_to_one = models.OneToOneField(
        'B', on_delete=models.SET_NULL, db_constraint=False, null=True
    )

    b_one_to_many_null = models.ForeignKey(
        'B', on_delete=models.SET_NULL, db_constraint=False, null=True,
        related_name='+'
    )
    b_one_to_many_nothing = models.ForeignKey(
        'B', on_delete=models.DO_NOTHING, db_constraint=False,
        related_name='+'
    )


class B(models.Model):
    pass

python manage.py makemigrations command generates following sql code

BEGIN;
--
-- Create model A
--
CREATE TABLE `a` (`id` integer AUTO_INCREMENT NOT NULL PRIMARY KEY);
--
-- Create model B
--
CREATE TABLE `b` (`id` integer AUTO_INCREMENT NOT NULL PRIMARY KEY);
--
-- Add field b_one_to_many_nothing to a
--
ALTER TABLE `a` ADD COLUMN `b_one_to_many_nothing_id` integer NOT NULL;
--
-- Add field b_one_to_many_null to a
--
ALTER TABLE `a` ADD COLUMN `b_one_to_many_null_id` integer NULL;
--
-- Add field b_one_to_one to a
--
ALTER TABLE `a` ADD COLUMN `b_one_to_one_id` integer NULL UNIQUE;
CREATE INDEX `a_b_one_to_many_nothing_id_4fca4209` ON `a` (`b_one_to_many_nothing_id`);
CREATE INDEX `a_b_one_to_many_null_id_e498aa17` ON `a` (`b_one_to_many_null_id`);
COMMIT;

I understand that Django might want to create an index for b_one_to_many_null field due to on_delete=models.SET_NULL setting, but I don't understand why there is a need for an index for b_one_to_many_nothing field.

Is there a way to turn this automatic index creation off or it's just a Django bug?

I'm using Django 1.11.5.