diff --git a/toonz/sources/include/tgeometry.h b/toonz/sources/include/tgeometry.h
index bc0185b..9542e3e 100644
--- a/toonz/sources/include/tgeometry.h
+++ b/toonz/sources/include/tgeometry.h
@@ -192,7 +192,7 @@ public:
   inline explicit T3DPointT(const T4DPointT<T> &p);
 
   inline TPointT<T>& xy() { return *(TPointT<T>*)this; }
-  inline TPointT<T>& xy() const { return *(const TPointT<T>*)this; }
+  inline const TPointT<T>& xy() const { return *(const TPointT<T>*)this; }
 
   inline T3DPointT &operator+=(const T3DPointT &a)
     { return x += a.x, y += a.y, z += a.z, *this; }
@@ -301,8 +301,8 @@ public:
   inline TPointT<T>& xy() { return *(TPointT<T>*)this; }
   inline T3DPointT<T>& xyz() { return *(T3DPointT<T>*)this; }
 
-  inline TPointT<T>& xy() const { return *(const TPointT<T>*)this; }
-  inline T3DPointT<T>& xyz() const { return *(const T3DPointT<T>*)this; }
+  inline const TPointT<T>& xy() const { return *(const TPointT<T>*)this; }
+  inline const T3DPointT<T>& xyz() const { return *(const T3DPointT<T>*)this; }
   
   inline bool operator==(const T4DPointT &p) const
     { return x == p.x && y == p.y && z == p.z && w == p.w; }