diff --git a/package-lock.json b/package-lock.json
index dc1dc9dd..59abe522 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,12 +1,12 @@
 {
   "name": "arx-level-generator",
-  "version": "19.0.0",
+  "version": "20.0.0-alpha.0",
   "lockfileVersion": 2,
   "requires": true,
   "packages": {
     "": {
       "name": "arx-level-generator",
-      "version": "19.0.0",
+      "version": "20.0.0-alpha.0",
       "license": "GPL-3.0-or-later",
       "dependencies": {
         "arx-header-size": "^2.2.0",
diff --git a/package.json b/package.json
index ea09329b..da261951 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "arx-level-generator",
-  "version": "19.0.0",
+  "version": "20.0.0-alpha.0",
   "description": "A tool for creating Arx Fatalis maps",
   "main": "dist/src/index.js",
   "types": "dist/src/index.d.ts",
diff --git a/src/prefabs/rooms/Rooms.ts b/src/prefabs/rooms/Rooms.ts
index 9bf7a3d0..e22c7ece 100644
--- a/src/prefabs/rooms/Rooms.ts
+++ b/src/prefabs/rooms/Rooms.ts
@@ -12,8 +12,8 @@ function union(map1: ArxMap, map2: ArxMap) {
   // TODO: this removes both polygons when they overlap, which is ideal for walls
   // but not for ceilings and floors
 
-  const map1BB = map1.getBoundingBox()
-  const map2BB = map2.getBoundingBox()
+  const map1BB = map1.polygons.getBoundingBox()
+  const map2BB = map2.polygons.getBoundingBox()
 
   if (!map1BB.intersectsBox(map2BB)) {
     return