diff --git a/mapmaker/flatmap/layers.py b/mapmaker/flatmap/layers.py
index 0fef2b2a..69cc8c45 100644
--- a/mapmaker/flatmap/layers.py
+++ b/mapmaker/flatmap/layers.py
@@ -408,6 +408,8 @@ def add_group_features(self, group_name: str, features: list[Feature],
                     region_properties.pop('userdata', None)
                     for region in filter(lambda p: prepared_polygon.contains(p.geometry), regions):
                         region_properties.update(region.properties)
+                        if 'id' not in region_properties:
+                            region_properties.update({'id': f'{self.id}_region_{region.geojson_id}'})
                         feature = self.flatmap.new_feature(self.id, polygon, region_properties)
                         if feature is not None:
                             layer_features.append(feature)