diff --git a/src/instruments/blobMask.ts b/src/instruments/blobMask.ts
index 6bd4d96d643be87b7291eeae42f28ef48b76ab08..28027502328cc7ab74fcf678ee752e0b542ea0d3 100644
--- a/src/instruments/blobMask.ts
+++ b/src/instruments/blobMask.ts
@@ -64,6 +64,7 @@ export class BlobMask extends AbstractInstrument<PathCoords> implements Instrume
             convexHull.strokeColor = new paper.Color("red");
             convexHull.strokeWidth = PaperUtils.absoluteDimension(2);
             convexHull.dashArray = [10, 12];
+            convexHull.closed = true;
             this.drawGroup.addChild(convexHull);
         }