diff --git a/src/string.c b/src/string.c
index 0d5ace186..b7e3a6504 100644
--- a/src/string.c
+++ b/src/string.c
@@ -3057,7 +3057,7 @@ mrb_init_string(mrb_state *mrb)
   mrb_static_assert(RSTRING_EMBED_LEN_MAX < (1 << MRB_STR_EMBED_LEN_BIT),
                     "pointer size too big for embedded string");
 
-  mrb->string_class = s = mrb_define_class(mrb, "String", mrb->object_class);             /* 15.2.10 */
+  mrb->string_class = s = mrb_define_class_id(mrb, MRB_SYM(String), mrb->object_class);             /* 15.2.10 */
   MRB_SET_INSTANCE_TT(s, MRB_TT_STRING);
 
   mrb_define_method_id(mrb, s, MRB_SYM(bytesize),        mrb_str_bytesize,        MRB_ARGS_NONE());