diff --git a/.github/workflows/test-python.yml b/.github/workflows/test-python.yml
index cc791d55..67062317 100644
--- a/.github/workflows/test-python.yml
+++ b/.github/workflows/test-python.yml
@@ -32,8 +32,8 @@ jobs:
       - name: Checkout Django
         uses: actions/checkout@v4
         with:
-          repository: 'mongodb-forks/django'
-          ref: 'mongodb-5.1.x'
+          repository: 'timgraham/django'
+          ref: 'objectid-no-int'
           path: 'django_repo'
           persist-credentials: false
       - name: Install system packages for Django's Python test dependencies
diff --git a/django_mongodb_backend/fields/auto.py b/django_mongodb_backend/fields/auto.py
index 5bf84e67..50f3b5b8 100644
--- a/django_mongodb_backend/fields/auto.py
+++ b/django_mongodb_backend/fields/auto.py
@@ -22,19 +22,11 @@ def deconstruct(self):
         return name, path, args, kwargs
 
     def get_prep_value(self, value):
-        if value is None:
-            return None
-        # Accept int for compatibility with Django's test suite which has many
-        # instances of manually assigned integer IDs, as well as for things
-        # like settings.SITE_ID which has a system check requiring an integer.
-        if isinstance(value, (ObjectId | int)):
+        if value is None or isinstance(value, ObjectId):
             return value
         try:
             return ObjectId(value)
         except errors.InvalidId as e:
-            # A manually assigned integer ID?
-            if isinstance(value, str) and value.isdigit():
-                return int(value)
             raise ValueError(f"Field '{self.name}' expected an ObjectId but got {value!r}.") from e
 
     def get_internal_type(self):
@@ -46,14 +38,11 @@ def to_python(self, value):
         try:
             return ObjectId(value)
         except errors.InvalidId:
-            try:
-                return int(value)
-            except ValueError:
-                raise exceptions.ValidationError(
-                    self.error_messages["invalid"],
-                    code="invalid",
-                    params={"value": value},
-                ) from None
+            raise exceptions.ValidationError(
+                self.error_messages["invalid"],
+                code="invalid",
+                params={"value": value},
+            ) from None
 
     @cached_property
     def validators(self):
diff --git a/tests/model_fields_/test_autofield.py b/tests/model_fields_/test_autofield.py
index 1ba2edc4..7d4b81ce 100644
--- a/tests/model_fields_/test_autofield.py
+++ b/tests/model_fields_/test_autofield.py
@@ -1,3 +1,4 @@
+from django.core.exceptions import ValidationError
 from django.test import SimpleTestCase
 
 from django_mongodb_backend.fields import ObjectIdAutoField
@@ -17,4 +18,6 @@ def test_get_internal_type(self):
 
     def test_to_python(self):
         f = ObjectIdAutoField()
-        self.assertEqual(f.to_python("1"), 1)
+        msg = "“1” is not a valid Object Id."
+        with self.assertRaisesMessage(ValidationError, msg):
+            f.to_python("1")