diff --git a/backend.go b/backend.go
index 5300aae..850c50a 100644
--- a/backend.go
+++ b/backend.go
@@ -5,7 +5,7 @@ import "image"
 import "errors"
 
 type Backend interface {
-	NewWindow       (image.Rectangle) MainWindow
+	NewWindow       (image.Rectangle) (MainWindow, error)
 	NewBox          () Box
 	NewTextBox      () TextBox
 	NewCanvasBox    () CanvasBox