ORM: [fieldname]__in when passed a string iterates over digits of the string

[Christian Ledermann]

Can be reproduced in a virgin new project with django-admin startproject mysite
Django version 1.11.13, python version 2.7
Databases tested: PostgreSQL and sqlite3

Observed behaviour:

(Pdb) from django.contrib.auth import get_user_model
(Pdb) User = get_user_model()
(Pdb) query = User.objects.filter(id__in='1234567890')
(Pdb) print query.query
SELECT "auth_user"."id", "auth_user"."password", "auth_user"."last_login", "auth_user"."is_superuser", "auth_user"."username", "auth_user"."first_name", "auth_user"."last_name", "auth_user"."email", "auth_user"."is_staff", "auth_user"."is_active", "auth_user"."date_joined" FROM "auth_user" WHERE "auth_user"."id" IN (0, 1, 2, 3, 4, 5, 6, 7, 8, 9)

Expected Behaviour:
An error is raised