diff --git a/mrbgems/mruby-string-ext/src/string.c b/mrbgems/mruby-string-ext/src/string.c
index 1fdb20127..50f58a468 100644
--- a/mrbgems/mruby-string-ext/src/string.c
+++ b/mrbgems/mruby-string-ext/src/string.c
@@ -1321,7 +1321,7 @@ str_ascii_only_p(mrb_state *mrb, mrb_value str)
     if (*p & 0x80) return mrb_false_value();
     p++;
   }
-  RSTR_SET_SINGLE_BYTE_FLAG(mrb_str_ptr(str));
+  mrb_str_ptr(str)->flags |= MRB_STR_SINGLE_BYTE;
   return mrb_true_value();
 }