diff --git a/package.json b/package.json
index e35de83..836b25f 100644
--- a/package.json
+++ b/package.json
@@ -28,6 +28,7 @@
   "dependencies": {
     "@turf/helpers": "^7.1.0",
     "@turf/union": "^7.1.0",
+    "@types/geojson": "^7946.0.14",
     "accessible-autocomplete": "^2.0.4",
     "file-saver": "^2.0.5",
     "govuk-frontend": "^5.7.1",
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 507bcf8..fa9c5c8 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -14,6 +14,9 @@ dependencies:
   '@turf/union':
     specifier: ^7.1.0
     version: 7.1.0
+  '@types/geojson':
+    specifier: ^7946.0.14
+    version: 7946.0.14
   accessible-autocomplete:
     specifier: ^2.0.4
     version: 2.0.4
@@ -228,7 +231,7 @@ packages:
       '@babel/core': 7.24.7
       '@babel/helper-compilation-targets': 7.24.7
       '@babel/helper-plugin-utils': 7.24.7
-      debug: 4.3.5
+      debug: 4.3.7
       lodash.debounce: 4.0.8
       resolve: 1.22.8
     transitivePeerDependencies:
@@ -6675,7 +6678,7 @@ packages:
     resolution: {integrity: sha512-v67WcEouB5GxbTWL/4NeToqcZiAWEq90N888fczVArY8A79J0L4FD7vj5hm3eUMua5EpoQ59wa/oovY6TLvRUA==}
     engines: {node: '>= 18'}
     dependencies:
-      debug: 4.3.5
+      debug: 4.3.7
       destroy: 1.2.0
       encodeurl: 2.0.0
       escape-html: 1.0.3
@@ -6943,7 +6946,7 @@ packages:
   /spdy-transport@3.0.0:
     resolution: {integrity: sha512-hsLVFE5SjA6TCisWeJXFKniGGOpBgMLmerfO2aCyCU5s7nJ/rpAepqmFifv/GCbSbueEeAJJnmSQ2rKC/g8Fcw==}
     dependencies:
-      debug: 4.3.5
+      debug: 4.3.7
       detect-node: 2.1.0
       hpack.js: 2.1.6
       obuf: 1.1.2
diff --git a/src/components/my-map/os-features.ts b/src/components/my-map/os-features.ts
index b0474d3..499b835 100644
--- a/src/components/my-map/os-features.ts
+++ b/src/components/my-map/os-features.ts
@@ -8,6 +8,7 @@ import { getServiceURL } from "../../lib/ordnanceSurvey";
 
 import { hexToRgba } from "./utils";
 import { featureCollection } from "@turf/helpers";
+import { MultiPolygon, Polygon, Feature } from "geojson";
 
 const featureSource = new VectorSource();
 
@@ -120,18 +121,22 @@ export function getFeaturesAtPoint(
       }
 
       outlineSource.clear();
-      outlineSource.addFeature(
-        // Merge all of the features into a single feature
-        geojson.readFeature(
-          featureSource.getFeatures().reduce((acc: any, curr) => {
-            const currentFeature = geojson.writeFeatureObject(curr);
-            if (!acc) return currentFeature;
-
-            const merged = union(featureCollection([currentFeature, acc]));
-            return merged;
-          }, null),
-        ),
-      );
+
+      // Convert OL features to GeoJSON
+      const allFeatures = featureSource
+        .getFeatures()
+        .map((feature) => geojson.writeFeatureObject(feature))
+        .filter((feature): feature is Feature<Polygon | MultiPolygon> =>
+          ["Polygon", "MultiPolygon"].includes(feature.geometry.type),
+        );
+
+      // Merge all GeoJSON features
+      const collection = featureCollection(allFeatures);
+      const mergedGeoJSON = union(collection);
+
+      // Convert back to OL feature and add to source
+      const mergedFeature = geojson.readFeature(mergedGeoJSON);
+      outlineSource.addFeature(mergedFeature);
     })
     .catch((error) => console.log(error));
 }