diff --git a/include/gdiplusflat.h b/include/gdiplusflat.h
index 5fc4ed59a2b..cd97a70332e 100644
--- a/include/gdiplusflat.h
+++ b/include/gdiplusflat.h
@@ -90,6 +90,7 @@ GpStatus WINGDIPAPI GdipSetCustomLineCapStrokeCaps(GpCustomLineCap*,GpLineCap,
 GpStatus WINGDIPAPI GdipGetCustomLineCapStrokeJoin(GpCustomLineCap*,GpLineJoin*);
 GpStatus WINGDIPAPI GdipSetCustomLineCapStrokeJoin(GpCustomLineCap*,GpLineJoin);
 GpStatus WINGDIPAPI GdipGetCustomLineCapWidthScale(GpCustomLineCap*,REAL*);
+GpStatus WINGDIPAPI GdipSetCustomLineCapWidthScale(GpCustomLineCap*,REAL);
 GpStatus WINGDIPAPI GdipSetCustomLineCapBaseInset(GpCustomLineCap*,REAL);
 
 /* Font */