diff --git a/src/error.c b/src/error.c
index 430728db0..b9e5690a8 100644
--- a/src/error.c
+++ b/src/error.c
@@ -418,8 +418,6 @@ mrb_init_exception(mrb_state *mrb)
   mrb_define_method(mrb, e, "inspect", exc_inspect, ARGS_NONE());
 
   mrb->eStandardError_class     = mrb_define_class(mrb, "StandardError",       mrb->eException_class); /* 15.2.23 */
-  mrb_define_class(mrb, "RuntimeError", mrb->eStandardError_class);                                    /* 15.2.28 */
-
   mrb_define_class(mrb, "RuntimeError", mrb->eStandardError_class);                                    /* 15.2.28 */
   e = mrb_define_class(mrb, "ScriptError",  mrb->eException_class);                                    /* 15.2.37 */
   mrb_define_class(mrb, "SyntaxError",  e);                                                            /* 15.2.38 */