diff --git a/src/kiri/mode/cam/prepare.js b/src/kiri/mode/cam/prepare.js
index 8c1bd591..da3d05b7 100644
--- a/src/kiri/mode/cam/prepare.js
+++ b/src/kiri/mode/cam/prepare.js
@@ -553,7 +553,7 @@ export async function prepare_one(widget, settings, print, firstPoint, update) {
 
     function setTravelBoundary(polys) {
         lastTravelBounds = travelBounds;
-        travelBounds = POLY.flatten(polys);
+        travelBounds = polys ? POLY.flatten(polys) : undefined;
     }
 
     /**