diff --git a/mrbgems/mruby-string-ext/src/string.c b/mrbgems/mruby-string-ext/src/string.c
index 31de0109c..1fdb20127 100644
--- a/mrbgems/mruby-string-ext/src/string.c
+++ b/mrbgems/mruby-string-ext/src/string.c
@@ -1328,7 +1328,7 @@ str_ascii_only_p(mrb_state *mrb, mrb_value str)
 static mrb_value
 str_b(mrb_state *mrb, mrb_value str)
 {
-  RSTR_SET_BINARY_FLAG(mrb_str_ptr(str));
+  mrb_str_ptr(str)->flags |= MRB_STR_BINARY;
   return str;
 }