diff --git a/src/error.c b/src/error.c
index 7f6d07a62..c91725283 100644
--- a/src/error.c
+++ b/src/error.c
@@ -665,24 +665,24 @@ mrb_init_exception(mrb_state *mrb)
 {
   struct RClass *exception, *script_error, *stack_error, *nomem_error;
 
-  mrb->eException_class = exception = mrb_define_class(mrb, "Exception", mrb->object_class); /* 15.2.22 */
+  mrb->eException_class = exception = mrb_define_class_id(mrb, MRB_SYM(Exception), mrb->object_class); /* 15.2.22 */
   MRB_SET_INSTANCE_TT(exception, MRB_TT_EXCEPTION);
-  mrb_define_class_method(mrb, exception, "exception", mrb_instance_new,  MRB_ARGS_OPT(1));
-  mrb_define_method(mrb, exception, "exception",       exc_exception,     MRB_ARGS_OPT(1));
-  mrb_define_method(mrb, exception, "initialize",      exc_initialize,    MRB_ARGS_OPT(1));
-  mrb_define_method(mrb, exception, "to_s",            exc_to_s,          MRB_ARGS_NONE());
-  mrb_define_method(mrb, exception, "inspect",         mrb_exc_inspect,   MRB_ARGS_NONE());
-  mrb_define_method(mrb, exception, "backtrace",       mrb_exc_backtrace, MRB_ARGS_NONE());
-  mrb_define_method(mrb, exception, "set_backtrace",   exc_set_backtrace, MRB_ARGS_REQ(1));
+  mrb_define_class_method_id(mrb, exception, MRB_SYM(exception), mrb_instance_new,  MRB_ARGS_OPT(1));
+  mrb_define_method_id(mrb, exception, MRB_SYM(exception),       exc_exception,     MRB_ARGS_OPT(1));
+  mrb_define_method_id(mrb, exception, MRB_SYM(initialize),      exc_initialize,    MRB_ARGS_OPT(1));
+  mrb_define_method_id(mrb, exception, MRB_SYM(to_s),            exc_to_s,          MRB_ARGS_NONE());
+  mrb_define_method_id(mrb, exception, MRB_SYM(inspect),         mrb_exc_inspect,   MRB_ARGS_NONE());
+  mrb_define_method_id(mrb, exception, MRB_SYM(backtrace),       mrb_exc_backtrace, MRB_ARGS_NONE());
+  mrb_define_method_id(mrb, exception, MRB_SYM(set_backtrace),   exc_set_backtrace, MRB_ARGS_REQ(1));
 
-  mrb->eStandardError_class = mrb_define_class(mrb, "StandardError", mrb->eException_class); /* 15.2.23 */
-  mrb_define_class(mrb, "RuntimeError", E_STANDARD_ERROR);          /* 15.2.28 */
-  script_error = mrb_define_class(mrb, "ScriptError", exception);                /* 15.2.37 */
-  mrb_define_class(mrb, "SyntaxError", script_error);                                        /* 15.2.38 */
-  stack_error = mrb_define_class(mrb, "SystemStackError", exception);
+  mrb->eStandardError_class = mrb_define_class_id(mrb, MRB_SYM(StandardError), mrb->eException_class); /* 15.2.23 */
+  mrb_define_class_id(mrb, MRB_SYM(RuntimeError), E_STANDARD_ERROR);          /* 15.2.28 */
+  script_error = mrb_define_class_id(mrb, MRB_SYM(ScriptError), exception);   /* 15.2.37 */
+  mrb_define_class_id(mrb, MRB_SYM(SyntaxError), script_error);               /* 15.2.38 */
+  stack_error = mrb_define_class_id(mrb, MRB_SYM(SystemStackError), exception);
   mrb->stack_err = mrb_obj_ptr(mrb_exc_new_lit(mrb, stack_error, "stack level too deep"));
 
-  nomem_error = mrb_define_class(mrb, "NoMemoryError", exception);
+  nomem_error = mrb_define_class_id(mrb, MRB_SYM(NoMemoryError), exception);
   mrb->nomem_err = mrb_obj_ptr(mrb_exc_new_lit(mrb, nomem_error, "Out of memory"));
 #ifdef MRB_GC_FIXED_ARENA
   mrb->arena_err = mrb_obj_ptr(mrb_exc_new_lit(mrb, nomem_error, "arena overflow error"));