diff --git a/django/db/models/fields/related_descriptors.py b/django/db/models/fields/related_descriptors.py
index 8468a0b01fa..ec603bc8afd 100644
--- a/django/db/models/fields/related_descriptors.py
+++ b/django/db/models/fields/related_descriptors.py
@@ -1064,7 +1064,7 @@ def create_forward_many_to_many_manager(superclass, rel, reverse):
                         (self.model._meta.object_name, obj)
                     )
                 else:
-                    target_ids.add(obj)
+                    target_ids.add(target_field.get_prep_value(obj))
             return target_ids
 
         def _get_missing_target_ids(self, source_field_name, target_field_name, db, target_ids):
diff --git a/tests/many_to_many/tests.py b/tests/many_to_many/tests.py
index a60ea424309..79d0fa420c2 100644
--- a/tests/many_to_many/tests.py
+++ b/tests/many_to_many/tests.py
@@ -1,7 +1,7 @@
 from unittest import mock
 
 from django.db import transaction
-from django.test import TestCase, skipUnlessDBFeature
+from django.test import TestCase, skipIfDBFeature, skipUnlessDBFeature
 
 from .models import (
     Article, InheritedArticleA, InheritedArticleB, Publication, User,
@@ -158,6 +158,14 @@ class ManyToManyTests(TestCase):
         with self.assertNumQueries(1):
             self.a1.publications.add(self.p1, self.p2)
 
+    @skipIfDBFeature('supports_ignore_conflicts')
+    def test_add_existing_different_type(self):
+        # A single SELECT query is necessary to compare existing values to the
+        # provided one; no INSERT should be attempted.
+        with self.assertNumQueries(1):
+            self.a1.publications.add(str(self.p1.pk))
+        self.assertEqual(self.a1.publications.get(), self.p1)
+
     @skipUnlessDBFeature('supports_ignore_conflicts')
     def test_slow_add_ignore_conflicts(self):
         manager_cls = self.a1.publications.__class__