diff --git a/mrbgems/mruby-sprintf/src/sprintf.c b/mrbgems/mruby-sprintf/src/sprintf.c
index 5d8b7678c..fa195f7c4 100644
--- a/mrbgems/mruby-sprintf/src/sprintf.c
+++ b/mrbgems/mruby-sprintf/src/sprintf.c
@@ -1097,7 +1097,7 @@ retry:
   if (posarg >= 0 && nextarg < argc) {
     const char *mesg = "too many arguments for format string";
     if (mrb_test(ruby_debug)) mrb_raise(mrb, E_ARGUMENT_ERROR, mesg);
-    if (mrb_test(ruby_verbose)) mrb_warn(mrb, "%s", mesg);
+    if (mrb_test(ruby_verbose)) mrb_warn(mrb, mesg);
   }
 #endif
   mrb_str_resize(mrb, result, blen);