diff --git a/mrbgems/mruby-string-ext/src/string.c b/mrbgems/mruby-string-ext/src/string.c
index fb35a8b9f..e8ea2c3ab 100644
--- a/mrbgems/mruby-string-ext/src/string.c
+++ b/mrbgems/mruby-string-ext/src/string.c
@@ -1352,8 +1352,9 @@ str_ascii_only_p(mrb_state *mrb, mrb_value str)
 }
 
 static mrb_value
-str_b(mrb_state *mrb, mrb_value str)
+str_b(mrb_state *mrb, mrb_value self)
 {
+  mrb_value str = mrb_str_dup(mrb, self);
   mrb_str_ptr(str)->flags |= MRB_STR_BINARY;
   return str;
 }