diff --git a/CHANGELOG.txt b/CHANGELOG.txt
index 86c0d28d4f..a07f6f5025 100644
--- a/CHANGELOG.txt
+++ b/CHANGELOG.txt
@@ -29,6 +29,7 @@ Changelog
  * Fix: Ensure `ImproperlyConfigured` is thrown from `db_field` on unbound `FieldPanel`s as intended (Matt Westcott)
  * Fix: Refine the positioning of the add comment button next to select, radio, checkbox fields and between field row columns (Srishti Jaiswal)
  * Fix: Show the correct privacy status for child collections of private collections (Shlomo Markowitz)
+ * Fix: Ensure reference index correctly handles models with primary keys not named `id` (Sage Abdullah)
  * Docs: Add missing `django.contrib.admin` to list of apps in "add to Django project" guide (Mohamed Rabiaa)
  * Docs: Add tutorial on deploying on Ubuntu to third-party tutorials (Mohammad Fathi Rahman)
  * Docs: Document that request_or_site is optional on BaseGenericSetting.load (Matt Westcott)
diff --git a/docs/releases/6.5.md b/docs/releases/6.5.md
index 3b171dc46f..526e7beaf0 100644
--- a/docs/releases/6.5.md
+++ b/docs/releases/6.5.md
@@ -44,6 +44,7 @@ This version adds formal support for Django 5.2.
  * Ensure `ImproperlyConfigured` is thrown from `db_field` on unbound `FieldPanel`s as intended (Matt Westcott)
  * Refine the positioning of the add comment button next to select, radio, checkbox fields and between field row columns (Srishti Jaiswal)
  * Show the correct privacy status for child collections of private collections (Shlomo Markowitz)
+ * Ensure reference index correctly handles models with primary keys not named `id` (Sage Abdullah)
 
 ### Documentation
 
diff --git a/wagtail/models/reference_index.py b/wagtail/models/reference_index.py
index a07109b93b..b7c8b5d2f9 100644
--- a/wagtail/models/reference_index.py
+++ b/wagtail/models/reference_index.py
@@ -384,7 +384,7 @@ class ReferenceIndex(models.Model):
                             to_content_type_id,
                             to_object_id,
                             f"{relation_name}.item.{model_path}",
-                            f"{relation_name}.{str(child_object.id)}.{content_path}",
+                            f"{relation_name}.{str(child_object.pk)}.{content_path}",
                         )
                         for to_content_type_id, to_object_id, model_path, content_path in cls._extract_references_from_object(
                             child_object
diff --git a/wagtail/test/testapp/migrations/0050_headcountrelatedmodelusingpk_related_page.py b/wagtail/test/testapp/migrations/0050_headcountrelatedmodelusingpk_related_page.py
new file mode 100644
index 0000000000..cb56096cb9
--- /dev/null
+++ b/wagtail/test/testapp/migrations/0050_headcountrelatedmodelusingpk_related_page.py
@@ -0,0 +1,25 @@
+# Generated by Django 6.0.dev20250303103700 on 2025-03-21 07:17
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("tests", "0049_promotionalpage"),
+        ("wagtailcore", "0094_alter_page_locale"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="headcountrelatedmodelusingpk",
+            name="related_page",
+            field=models.ForeignKey(
+                null=True,
+                on_delete=django.db.models.deletion.CASCADE,
+                related_name="head_count_relations",
+                to="wagtailcore.page",
+            ),
+        ),
+    ]
diff --git a/wagtail/test/testapp/models.py b/wagtail/test/testapp/models.py
index c776c42e46..082790706e 100644
--- a/wagtail/test/testapp/models.py
+++ b/wagtail/test/testapp/models.py
@@ -459,6 +459,12 @@ class HeadCountRelatedModelUsingPK(models.Model):
     event_page = ParentalKey(
         EventPage, on_delete=models.CASCADE, related_name="head_counts"
     )
+    related_page = models.ForeignKey(
+        Page,
+        on_delete=models.CASCADE,
+        related_name="head_count_relations",
+        null=True,
+    )
     head_count = models.IntegerField()
     panels = [FieldPanel("head_count")]
 
diff --git a/wagtail/tests/test_reference_index.py b/wagtail/tests/test_reference_index.py
index a3fc9d731a..ca471c186a 100644
--- a/wagtail/tests/test_reference_index.py
+++ b/wagtail/tests/test_reference_index.py
@@ -21,6 +21,7 @@ from wagtail.test.testapp.models import (
     GenericSnippetNoFieldIndexPage,
     GenericSnippetNoIndexPage,
     GenericSnippetPage,
+    HeadCountRelatedModelUsingPK,
     ModelWithNullableParentalKey,
     VariousOnDeleteModel,
 )
@@ -281,6 +282,31 @@ class TestCreateOrUpdateForObject(TestCase):
         self.assertIn(" 3  wagtail.images.models.Image", stdout.getvalue())
         self.assertIn(" 4  wagtail.test.testapp.models.EventPage", stdout.getvalue())
 
+    def test_inline_custom_pk_model(self):
+        related_page = EventPage(
+            title="Related page",
+            slug="related-page",
+            location="the moon",
+            audience="public",
+            cost="free",
+            date_from="2025-03-21",
+        )
+        with self.captureOnCommitCallbacks(execute=True):
+            self.root_page.add_child(instance=related_page)
+
+        refs = ReferenceIndex.get_references_to(related_page)
+        self.assertEqual(refs.count(), 0)
+
+        with self.captureOnCommitCallbacks(execute=True):
+            HeadCountRelatedModelUsingPK.objects.create(
+                head_count=1234,
+                event_page=self.event_page,
+                related_page=related_page,
+            )
+
+        refs = ReferenceIndex.get_references_to(related_page)
+        self.assertEqual(refs.count(), 1)
+
 
 class TestDescribeOnDelete(TestCase):
     fixtures = ["test.json"]