diff --git a/mrbgems/mruby-sprintf/src/sprintf.c b/mrbgems/mruby-sprintf/src/sprintf.c
index 742056371..f2caada22 100644
--- a/mrbgems/mruby-sprintf/src/sprintf.c
+++ b/mrbgems/mruby-sprintf/src/sprintf.c
@@ -1182,8 +1182,8 @@ void
 mrb_mruby_sprintf_gem_init(mrb_state *mrb)
 {
   struct RClass *krn = mrb->kernel_module;
-  mrb_define_module_function(mrb, krn, "sprintf", mrb_f_sprintf, MRB_ARGS_ANY());
-  mrb_define_module_function(mrb, krn, "format",  mrb_f_sprintf, MRB_ARGS_ANY());
+  mrb_define_module_function_id(mrb, krn, MRB_SYM(sprintf), mrb_f_sprintf, MRB_ARGS_ANY());
+  mrb_define_module_function_id(mrb, krn, MRB_SYM(format),  mrb_f_sprintf, MRB_ARGS_ANY());
 }
 
 void