diff --git a/mrbgems/mruby-string-ext/src/string.c b/mrbgems/mruby-string-ext/src/string.c
index 89ff4df82..4e2be81ff 100644
--- a/mrbgems/mruby-string-ext/src/string.c
+++ b/mrbgems/mruby-string-ext/src/string.c
@@ -1071,7 +1071,7 @@ str_codepoints(mrb_state *mrb, mrb_value self)
 {
   mrb_value result;
   char *p = RSTRING_PTR(self);
-  char *e = b + RSTRING_LEN(self);
+  char *e = p + RSTRING_LEN(self);
 
   mrb->c->ci->mid = 0;
   result = mrb_ary_new(mrb);