diff --git a/wagtail/signal_handlers.py b/wagtail/signal_handlers.py
index 8431843f5f..1695c67f10 100644
--- a/wagtail/signal_handlers.py
+++ b/wagtail/signal_handlers.py
@@ -83,6 +83,9 @@ def update_reference_index_on_save(instance, **kwargs):
             break
 
         instance = getattr(instance, parental_keys[0].name)
+        if instance is None:
+            # parent is null, so there is no valid object to record references against
+            return
 
     if ReferenceIndex.model_is_indexable(type(instance)):
         with transaction.atomic():
diff --git a/wagtail/test/testapp/migrations/0011_modelwithnullableparentalkey.py b/wagtail/test/testapp/migrations/0011_modelwithnullableparentalkey.py
new file mode 100644
index 0000000000..1ad92ddd94
--- /dev/null
+++ b/wagtail/test/testapp/migrations/0011_modelwithnullableparentalkey.py
@@ -0,0 +1,41 @@
+# Generated by Django 4.1.2 on 2022-11-04 14:00
+
+from django.db import migrations, models
+import django.db.models.deletion
+import modelcluster.fields
+import wagtail.fields
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("wagtailcore", "0078_referenceindex"),
+        ("tests", "0010_alter_customimage_file_and_more"),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name="ModelWithNullableParentalKey",
+            fields=[
+                (
+                    "id",
+                    models.AutoField(
+                        auto_created=True,
+                        primary_key=True,
+                        serialize=False,
+                        verbose_name="ID",
+                    ),
+                ),
+                ("content", wagtail.fields.RichTextField()),
+                (
+                    "page",
+                    modelcluster.fields.ParentalKey(
+                        blank=True,
+                        null=True,
+                        on_delete=django.db.models.deletion.CASCADE,
+                        to="wagtailcore.page",
+                    ),
+                ),
+            ],
+        ),
+    ]
diff --git a/wagtail/test/testapp/models.py b/wagtail/test/testapp/models.py
index 33ee9b2017..1f243b3355 100644
--- a/wagtail/test/testapp/models.py
+++ b/wagtail/test/testapp/models.py
@@ -1975,3 +1975,14 @@ class ModelWithStringTypePrimaryKey(models.Model):
 
     custom_id = models.CharField(max_length=255, primary_key=True)
     content = models.CharField(max_length=255)
+
+
+class ModelWithNullableParentalKey(models.Model):
+    """
+    There's not really a valid use case for null parental keys, but their presence should not
+    break things outright (e.g. when determining the object ID to store things under in the
+    references index).
+    """
+
+    page = ParentalKey(Page, blank=True, null=True)
+    content = RichTextField()
diff --git a/wagtail/tests/test_reference_index.py b/wagtail/tests/test_reference_index.py
index 10c78e87c7..30089d0830 100644
--- a/wagtail/tests/test_reference_index.py
+++ b/wagtail/tests/test_reference_index.py
@@ -7,7 +7,11 @@ from django.test import TestCase
 from wagtail.images import get_image_model
 from wagtail.images.tests.utils import get_test_image_file
 from wagtail.models import Page, ReferenceIndex
-from wagtail.test.testapp.models import EventPage, EventPageCarouselItem
+from wagtail.test.testapp.models import (
+    EventPage,
+    EventPageCarouselItem,
+    ModelWithNullableParentalKey,
+)
 
 
 class TestCreateOrUpdateForObject(TestCase):
@@ -163,6 +167,18 @@ class TestCreateOrUpdateForObject(TestCase):
             self.expected_references,
         )
 
+    def test_null_parental_key(self):
+        obj = ModelWithNullableParentalKey(
+            content="""<p><a linktype="page" id="%d">event page</a></p>"""
+            % self.event_page.id
+        )
+        obj.save()
+
+        # Models with a ParentalKey are not considered indexable - references are recorded against the parent model
+        # instead. Since the ParentalKey is null here, no reference will be recorded.
+        refs = ReferenceIndex.get_references_to(self.event_page)
+        self.assertEqual(refs.count(), 0)
+
     def test_rebuild_references_index_no_verbosity(self):
         stdout = StringIO()
         management.call_command(