diff --git a/mrbgems/mruby-string-ext/src/string.c b/mrbgems/mruby-string-ext/src/string.c
index 4e2be81ff..0e3b56c2a 100644
--- a/mrbgems/mruby-string-ext/src/string.c
+++ b/mrbgems/mruby-string-ext/src/string.c
@@ -1002,8 +1002,8 @@ str_succ(mrb_state *mrb, mrb_value self)
 #ifdef MRB_UTF8_STRING
 extern const char mrb_utf8len_table[];
 
-static mrb_int
-utf8code(unsigned char* p, mrb_int limit)
+MRB_INLINE mrb_int
+utf8code(const unsigned char* p, const unsigned char *e)
 {
   mrb_int len;
 
@@ -1011,7 +1011,7 @@ utf8code(unsigned char* p, mrb_int limit)
     return p[0];
 
   len = mrb_utf8len_table[p[0]>>3];
-  if (len <= limit && len > 1 && (p[1] & 0xc0) == 0x80) {
+  if (p+len <= e && len > 1 && (p[1] & 0xc0) == 0x80) {
     if (len == 2)
       return ((p[0] & 0x1f) << 6) + (p[1] & 0x3f);
     if ((p[2] & 0xc0) == 0x80) {
@@ -1033,26 +1033,27 @@ str_ord(mrb_state* mrb, mrb_value str)
 {
   if (RSTRING_LEN(str) == 0)
     mrb_raise(mrb, E_ARGUMENT_ERROR, "empty string");
-  mrb_int c = utf8code((unsigned char*)RSTRING_PTR(str), RSTRING_LEN(str));
+  const unsigned char *p = (unsigned char*)RSTRING_PTR(str);
+  const unsigned char *e = p + RSTRING_LEN(str);
+
+  mrb_int c = utf8code(p, e);
   if (c < 0) mrb_raise(mrb, E_ARGUMENT_ERROR, "invalid UTF-8 byte sequence");
   return mrb_fixnum_value(c);
 }
 
 static mrb_value
-str_codepoints(mrb_state *mrb, mrb_value self)
+str_codepoints(mrb_state *mrb, mrb_value str)
 {
   mrb_value result;
-  char *p = RSTRING_PTR(self);
-  mrb_int len = RSTRING_LEN(self);
-  char *e = p + len;
+  const unsigned char *p = (unsigned char*)RSTRING_PTR(str);
+  const unsigned char *e = p + RSTRING_LEN(str);
 
   mrb->c->ci->mid = 0;
   result = mrb_ary_new(mrb);
   while (p < e) {
-    mrb_int c = utf8code((unsigned char*)p, len);
+    mrb_int c = utf8code((unsigned char*)p, e);
     mrb_ary_push(mrb, result, mrb_int_value(mrb, c));
     mrb_int ulen = mrb_utf8len(p, e);
-    len -= ulen;
     p += ulen;
   }
   return result;