diff --git a/string.c b/string.c
index e984330b9a..1a1b9fd6ed 100644
--- a/string.c
+++ b/string.c
@@ -1560,7 +1560,7 @@ rb_str_resurrect(VALUE str)
  *
  *  Argument +str+, if given, it must be a
  *  {String-convertible object}[doc/implicit_conversion_rdoc.html#label-String-Convertible+Objects]
- *  (implements +to_str).
+ *  (implements +to_str+).
  *
  *  Argument +encoding+, if given, must be:
  *  - A {String-convertible object}[doc/implicit_conversion_rdoc.html#label-String-Convertible+Objects]