diff --git a/window.go b/window.go
index cf4d8f76a19504979453fd79bfe4c82aa724d859..177ea73550374758d186181458fe6bcc70597092 100644
--- a/window.go
+++ b/window.go
@@ -367,7 +367,7 @@ type windowTriangles struct {
 }
 
 func (wt *windowTriangles) Len() int {
-	return wt.vs.Len()
+	return len(wt.data)
 }
 
 func (wt *windowTriangles) Draw() {