diff --git a/AppKit/CPWindow/CPWindow.j b/AppKit/CPWindow/CPWindow.j
index 58f051a88..ab5c48ba3 100644
--- a/AppKit/CPWindow/CPWindow.j
+++ b/AppKit/CPWindow/CPWindow.j
@@ -581,7 +581,7 @@ CPTexturedBackgroundWindowMask
 */
 + (CGRect)frameRectForContentRect:(CGRect)aContentRect styleMask:(unsigned)aStyleMask
 {
-    return [[[self class] _windowViewClassForStyleMask:_styleMask] frameRectForContentRect:aContentRect];
+    return [[[self class] _windowViewClassForStyleMask:aStyleMask] frameRectForContentRect:aContentRect];
 }
 
 /*!