diff --git a/internal/system/canvasbox.go b/internal/system/canvasbox.go
index c88b3e3..d63de6c 100644
--- a/internal/system/canvasbox.go
+++ b/internal/system/canvasbox.go
@@ -29,6 +29,7 @@ func (this *canvasBox) Invalidate () {
 }
 
 func (this *canvasBox) Draw (can canvas.Canvas) {
+	if can == nil { return }
 	this.box.Draw(can)
 	if this.userDrawer != nil {
 		this.userDrawer.Draw (