Cannot search for foreign key's pk in the admin anymore

[rm_]

On django 1.10.6 and python 2.7.9 given this Model and ModelAdmin:

class Bar(models.Model):
    name = models.CharField(
        max_length=256
    )

class Foo(models.Model):
    name = models.CharField(
        max_length=256
    )
    bar = models.ForeignKey(Bar)

class FooAdmin(admin.ModelAdmin):
    search_fields = ['bar__pk']

Each search returns:

File "/venv/local/lib/python2.7/site-packages/django/contrib/admin/views/main.py" in __init__
  78.         self.queryset = self.get_queryset(request)

File "/venv/local/lib/python2.7/site-packages/django/contrib/admin/views/main.py" in get_queryset
  346.         qs, search_use_distinct = self.model_admin.get_search_results(request, qs, self.query)

File "/venv/local/lib/python2.7/site-packages/django/contrib/admin/options.py" in get_search_results
  908.                     if lookup_needs_distinct(self.opts, search_spec):

File "/venv/local/lib/python2.7/site-packages/django/contrib/admin/utils.py" in lookup_needs_distinct
  32.         field = opts.get_field(field_name)

File "/venv/local/lib/python2.7/site-packages/django/db/models/options.py" in get_field
  619.             raise FieldDoesNotExist('%s has no field named %r' % (self.object_name, field_name))

Exception Type: FieldDoesNotExist at /admin/app/foo/
Exception Value: Bar has no field named u'pk'

Same code works fine in 1.8.16.

UPDATE: i've updated the test case as searching for the pk of the admin model does not work in 1.8.16 too