diff --git a/django/contrib/admin/checks.py b/django/contrib/admin/checks.py
index 286c1169a4..f000fc269b 100644
--- a/django/contrib/admin/checks.py
+++ b/django/contrib/admin/checks.py
@@ -468,7 +468,8 @@ class BaseModelAdminChecks(object):
         else:
             if field_name.startswith('-'):
                 field_name = field_name[1:]
-
+            if field_name == 'pk':
+                return []
             try:
                 model._meta.get_field(field_name)
             except FieldDoesNotExist:
diff --git a/tests/modeladmin/test_checks.py b/tests/modeladmin/test_checks.py
index 0c66544b6b..ad43f34837 100644
--- a/tests/modeladmin/test_checks.py
+++ b/tests/modeladmin/test_checks.py
@@ -796,7 +796,7 @@ class OrderingCheckTests(CheckTestCase):
 
     def test_valid_case(self):
         class TestModelAdmin(ModelAdmin):
-            ordering = ('name',)
+            ordering = ('name', 'pk')
 
         self.assertIsValid(TestModelAdmin, ValidationTestModel)