diff --git a/include/mruby/object.h b/include/mruby/object.h
index 495c34083..4be060078 100644
--- a/include/mruby/object.h
+++ b/include/mruby/object.h
@@ -13,7 +13,7 @@ extern "C" {
 
 #define MRUBY_OBJECT_HEADER \
   enum mrb_vtype tt:8; \
-  int color:3;\
+  unsigned int color:3;\
   unsigned int flags:21;\
   struct RClass *c;\
   struct RBasic *gcnext