diff --git a/include/mruby.h b/include/mruby.h
index f15098069..5feb8d633 100644
--- a/include/mruby.h
+++ b/include/mruby.h
@@ -1357,7 +1357,7 @@ MRB_API mrb_noreturn void mrb_name_error(mrb_state *mrb, mrb_sym id, const char
 MRB_API mrb_noreturn void mrb_frozen_error(mrb_state *mrb, void *frozen_obj);
 MRB_API mrb_noreturn void mrb_argnum_error(mrb_state *mrb, mrb_int argc, int min, int max);
 MRB_API void mrb_warn(mrb_state *mrb, const char *fmt, ...);
-MRB_API mrb_noreturn void mrb_bug(mrb_state *mrb, const char *fmt, ...);
+MRB_API mrb_noreturn void mrb_bug(mrb_state *mrb, const char *mesg);
 MRB_API void mrb_print_backtrace(mrb_state *mrb);
 MRB_API void mrb_print_error(mrb_state *mrb);
 /* function for `raisef` formatting */
diff --git a/src/error.c b/src/error.c
index 81b65211f..7f6d07a62 100644
--- a/src/error.c
+++ b/src/error.c
@@ -443,17 +443,12 @@ mrb_warn(mrb_state *mrb, const char *fmt, ...)
 }
 
 MRB_API mrb_noreturn void
-mrb_bug(mrb_state *mrb, const char *fmt, ...)
+mrb_bug(mrb_state *mrb, const char *mesg)
 {
 #ifndef MRB_NO_STDIO
-  va_list ap;
-  mrb_value str;
-
-  va_start(ap, fmt);
-  str = mrb_vformat(mrb, fmt, ap);
   fputs("bug: ", stderr);
-  fwrite(RSTRING_PTR(str), RSTRING_LEN(str), 1, stderr);
-  va_end(ap);
+  fputs(mesg, stderr);
+  fputs("\n", stderr);
 #endif
   exit(EXIT_FAILURE);
 }