diff --git a/mrbgems/mruby-binding/src/binding.c b/mrbgems/mruby-binding/src/binding.c
index b4ee14ab1..05a068a0e 100644
--- a/mrbgems/mruby-binding/src/binding.c
+++ b/mrbgems/mruby-binding/src/binding.c
@@ -402,7 +402,7 @@ mrb_mruby_binding_gem_init(mrb_state *mrb)
 
   mrb_define_private_method_id(mrb, mrb->kernel_module, MRB_SYM(binding), mrb_f_binding, MRB_ARGS_NONE());
 
-  mrb_define_method_id(mrb, binding, MRB_SYM(initialize_copy), binding_initialize_copy, MRB_ARGS_REQ(1));
+  mrb_define_private_method_id(mrb, binding, MRB_SYM(initialize_copy), binding_initialize_copy, MRB_ARGS_REQ(1));
   mrb_define_method_id(mrb, binding, MRB_SYM_Q(local_variable_defined), binding_local_variable_defined_p, MRB_ARGS_REQ(1));
   mrb_define_method_id(mrb, binding, MRB_SYM(local_variable_get), binding_local_variable_get, MRB_ARGS_REQ(1));
   mrb_define_method_id(mrb, binding, MRB_SYM(local_variable_set), binding_local_variable_set, MRB_ARGS_REQ(2));