diff --git a/django/db/models/base.py b/django/db/models/base.py
index a348cf6de7c8b..04c802a04bac0 100644
--- a/django/db/models/base.py
+++ b/django/db/models/base.py
@@ -678,6 +678,10 @@ def _perform_unique_checks(self, unique_checks):
                 f = self._meta.get_field(field_name)
                 lookup_value = getattr(self, f.attname)
                 if f.null and lookup_value is None:
+                    # no value, skip the lookup
+                    continue
+                if f.primary_key and not getattr(self, '_adding', False):
+                    # no need to check for unique primary key when editting 
                     continue
                 lookup_kwargs[str(field_name)] = lookup_value
 
diff --git a/tests/modeltests/validation/test_unique.py b/tests/modeltests/validation/test_unique.py
index 98c9615927098..2790d378c6574 100644
--- a/tests/modeltests/validation/test_unique.py
+++ b/tests/modeltests/validation/test_unique.py
@@ -1,6 +1,9 @@
 import unittest
 
-from models import CustomPKModel, UniqueTogetherModel, UniqueFieldsModel, UniqueForDateModel
+from django.conf import settings
+from django.db import connection
+
+from models import CustomPKModel, UniqueTogetherModel, UniqueFieldsModel, UniqueForDateModel, ModelToValidate
 
 class GetUniqueCheckTests(unittest.TestCase):
     def test_unique_fields_get_collected(self):
@@ -24,4 +27,26 @@ def test_unique_for_date_gets_picked_up(self):
         )
 
 
-
+class PerformUniqueChecksTest(unittest.TestCase):
+    def setUp(self):
+        self._old_debug, settings.DEBUG = settings.DEBUG, True
+        super(PerformUniqueChecksTest, self).setUp()
+
+    def tearDown(self):
+        settings.DEBUG = self._old_debug
+        super(PerformUniqueChecksTest, self).tearDown()
+
+    def test_primary_key_unique_check_performed_when_adding(self):
+        "Check#12132"
+        l = len(connection.queries)
+        mtv = ModelToValidate(number=10, name='Some Name')
+        setattr(mtv, '_adding', True)
+        mtv.clean()
+        self.assertEqual(l+1, len(connection.queries))
+
+    def test_primary_key_unique_check_not_performed_when_not_adding(self):
+        "Check#12132"
+        l = len(connection.queries)
+        mtv = ModelToValidate(number=10, name='Some Name')
+        mtv.clean()
+        self.assertEqual(l, len(connection.queries))
diff --git a/tests/modeltests/validation/tests.py b/tests/modeltests/validation/tests.py
index 25f9d0c005ccc..656e9d8216268 100644
--- a/tests/modeltests/validation/tests.py
+++ b/tests/modeltests/validation/tests.py
@@ -5,7 +5,7 @@
 from models import *
 
 from validators import TestModelsWithValidators
-from test_unique import GetUniqueCheckTests
+from test_unique import GetUniqueCheckTests, PerformUniqueChecksTest
 from test_custom_messages import CustomMessagesTest
 
 class BaseModelValidationTests(ValidationTestCase):