diff --git a/src/proc.c b/src/proc.c
index 1b31d4e98..129843aa3 100644
--- a/src/proc.c
+++ b/src/proc.c
@@ -547,7 +547,7 @@ mrb_init_proc(mrb_state *mrb)
   MRB_SET_INSTANCE_TT(pc, MRB_TT_PROC);
   MRB_UNDEF_ALLOCATOR(pc);
   mrb_define_class_method_id(mrb, pc, MRB_SYM(new), mrb_proc_s_new, MRB_ARGS_NONE()|MRB_ARGS_BLOCK());
-  mrb_define_method_id(mrb, pc, MRB_SYM(initialize_copy), mrb_proc_init_copy, MRB_ARGS_REQ(1));
+  mrb_define_private_method_id(mrb, pc, MRB_SYM(initialize_copy), mrb_proc_init_copy, MRB_ARGS_REQ(1));
   mrb_define_method_id(mrb, pc, MRB_SYM(arity), proc_arity, MRB_ARGS_NONE()); /* 15.2.17.4.2 */
   mrb_define_method_id(mrb, pc, MRB_OPSYM(eq), proc_eql, MRB_ARGS_REQ(1));
   mrb_define_method_id(mrb, pc, MRB_SYM_Q(eql), proc_eql, MRB_ARGS_REQ(1));