diff --git a/src/string.c b/src/string.c
index bea939201..11ae5a318 100644
--- a/src/string.c
+++ b/src/string.c
@@ -3516,7 +3516,7 @@ mrb_init_string(mrb_state *mrb)
   mrb_define_method_id(mrb, s, MRB_SYM_Q(include),       mrb_str_include,         MRB_ARGS_REQ(1)); /* 15.2.10.5.21 */
   mrb_define_method_id(mrb, s, MRB_SYM(index),           mrb_str_index_m,         MRB_ARGS_ARG(1,1));  /* 15.2.10.5.22 */
   mrb_define_method_id(mrb, s, MRB_SYM(initialize),      mrb_str_init,            MRB_ARGS_REQ(1)); /* 15.2.10.5.23 */
-  mrb_define_method_id(mrb, s, MRB_SYM(initialize_copy), mrb_str_replace,         MRB_ARGS_REQ(1)); /* 15.2.10.5.24 */
+  mrb_define_private_method_id(mrb, s, MRB_SYM(initialize_copy), mrb_str_replace, MRB_ARGS_REQ(1)); /* 15.2.10.5.24 */
   mrb_define_method_id(mrb, s, MRB_SYM(intern),          mrb_str_intern,          MRB_ARGS_NONE()); /* 15.2.10.5.25 */
   mrb_define_method_id(mrb, s, MRB_SYM(length),          mrb_str_size,            MRB_ARGS_NONE()); /* 15.2.10.5.26 */
   mrb_define_method_id(mrb, s, MRB_SYM(replace),         mrb_str_replace,         MRB_ARGS_REQ(1)); /* 15.2.10.5.28 */