diff --git a/binding-mri/plane-binding.cpp b/binding-mri/plane-binding.cpp
index b1bf840..f203818 100644
--- a/binding-mri/plane-binding.cpp
+++ b/binding-mri/plane-binding.cpp
@@ -45,7 +45,7 @@ RB_METHOD(planeInitialize)
 
 #define DISP_CLASS_NAME "plane"
 
-DEF_PROP_OBJ(Plane, Bitmap, Bitmap, "bitmap")
+DEF_PROP_OBJ_NIL(Plane, Bitmap, Bitmap, "bitmap")
 DEF_PROP_OBJ(Plane, Color, Color, "color")
 DEF_PROP_OBJ(Plane, Tone, Tone, "tone")