diff --git a/src/Perspex.SceneGraph/Size.cs b/src/Perspex.SceneGraph/Size.cs
index 4370b14bda..1e7acc0096 100644
--- a/src/Perspex.SceneGraph/Size.cs
+++ b/src/Perspex.SceneGraph/Size.cs
@@ -125,7 +125,7 @@ namespace Perspex
 
         public static Size operator -(Size size, Size toSubstract)
         {
-            return new Size(size._width + toSubstract._width, size._height + toSubstract._height);
+            return new Size(size._width - toSubstract._width, size._height - toSubstract._height);
         }
 
         /// <summary>