diff --git a/include/mruby/string.h b/include/mruby/string.h
index b88259452..22683b89a 100644
--- a/include/mruby/string.h
+++ b/include/mruby/string.h
@@ -342,6 +342,16 @@ MRB_API const char *mrb_string_value_cstr(mrb_state *mrb, mrb_value *str);
  */
 MRB_API mrb_value mrb_str_dup(mrb_state *mrb, mrb_value str);
 
+/**
+ * Returns a frozen string object.
+ * The string will be duplicated and frozen if it is not already frozen.
+ *
+ * @param mrb The current mruby state.
+ * @param str An original Ruby string.
+ * @return [mrb_value] Ruby frozen string.
+ */
+MRB_API mrb_value mrb_str_dup_frozen(mrb_state *mrb, mrb_value str);
+
 /**
  * Returns a symbol from a passed in Ruby string.
  *
diff --git a/mrbgems/mruby-sprintf/src/sprintf.c b/mrbgems/mruby-sprintf/src/sprintf.c
index 9d2900b58..5f4404cf9 100644
--- a/mrbgems/mruby-sprintf/src/sprintf.c
+++ b/mrbgems/mruby-sprintf/src/sprintf.c
@@ -386,7 +386,7 @@ mrb_str_format(mrb_state *mrb, mrb_int argc, const mrb_value *argv, mrb_value fm
      buffer, so this is O(1); String#replace on the original goes
      through str_replace which decrements the shared refcount, leaving
      our copy's buffer intact. */
-  fmt = mrb_str_dup(mrb, fmt);
+  fmt = mrb_str_dup_frozen(mrb, fmt);
   p = RSTRING_PTR(fmt);
   end = p + RSTRING_LEN(fmt);
   blen = 0;
diff --git a/src/string.c b/src/string.c
index 851a42f47..900ce079a 100644
--- a/src/string.c
+++ b/src/string.c
@@ -1358,6 +1358,16 @@ mrb_str_dup(mrb_state *mrb, mrb_value str)
   return str_replace(mrb, dup, s);
 }
 
+MRB_API mrb_value
+mrb_str_dup_frozen(mrb_state *mrb, mrb_value str)
+{
+  if (!mrb_frozen_p(mrb_basic_ptr(str))) {
+    str = mrb_str_dup(mrb, str);
+    mrb_basic_ptr(str)->frozen = TRUE;
+  }
+  return str;
+}
+
 enum str_convert_range {
   /* `beg` and `len` are byte unit in `0 ... str.bytesize` */
   STR_BYTE_RANGE_CORRECTED = 1,