diff --git a/app/models/broadcast_message.py b/app/models/broadcast_message.py
index b7e6c218e..c5d334e4c 100644
--- a/app/models/broadcast_message.py
+++ b/app/models/broadcast_message.py
@@ -1,6 +1,7 @@
 import itertools
 from datetime import datetime, timedelta
 
+from flask import current_app
 from notifications_utils.polygons import Polygons
 from notifications_utils.template import BroadcastPreviewTemplate
 from orderedset import OrderedSet
@@ -92,12 +93,19 @@ class BroadcastMessage(JSONModel):
         if 'ids' in self._dict['areas']:
             library_areas = self.get_areas(self.area_ids)
 
-            if len(library_areas) != len(self.area_ids):
-                raise RuntimeError(
-                    f'BroadcastMessage has {len(self.area_ids)} areas '
-                    f'but {len(library_areas)} found in the library'
+            if len(library_areas) == len(self.area_ids):
+                return library_areas
+            else:
+                # it's possible an old broadcast may refer to areas that
+                # are no longer part of our area libraries; in this case
+                # we should just treat the whole thing as a custom broadcast,
+                # which isn't great as our code doesn't support editing its
+                # areas, but we don't expect this to happen often
+                current_app.logger.warn(
+                    f'BroadcastMessage has {len(self._dict["areas"])} areas '
+                    f'but {len(library_areas)} found in the library. Treating '
+                    f'{self.id} as a custom broadcast.'
                 )
-            return library_areas
 
         polygons = self._dict['areas'].get('simple_polygons', [])
 
diff --git a/tests/app/models/test_broadcast_message.py b/tests/app/models/test_broadcast_message.py
index 4e2705b90..8dcce9711 100644
--- a/tests/app/models/test_broadcast_message.py
+++ b/tests/app/models/test_broadcast_message.py
@@ -1,5 +1,6 @@
 import pytest
 
+from app.broadcast_areas.models import CustomBroadcastAreas
 from app.models.broadcast_message import BroadcastMessage
 from tests import broadcast_message_json
 
@@ -70,17 +71,23 @@ def test_areas(
     assert len(list(broadcast_message.areas)) == expected_length
 
 
-def test_areas_raises_for_missing_areas():
+def test_areas_treats_missing_ids_as_custom_broadcast(notify_admin):
     broadcast_message = BroadcastMessage(broadcast_message_json(
-        area_ids=[
-            'wd20-E05009372',
-            'something else',
-        ],
+        areas={
+            'ids': [
+                'wd20-E05009372',
+                'something else',
+            ],
+            # although the IDs may no longer be usable, we can
+            # expect the broadcast to have names and polygons,
+            # which is enough to show the user something
+            'names': [
+                'wd20 name',
+                'something else name'
+            ],
+            'simple_polygons': [[[1, 2]]]
+        }
     ))
 
-    with pytest.raises(RuntimeError) as exception:
-        broadcast_message.areas
-
-    assert str(exception.value) == (
-        'BroadcastMessage has 2 areas but 1 found in the library'
-    )
+    assert len(list(broadcast_message.areas)) == 2
+    assert type(broadcast_message.areas) == CustomBroadcastAreas