diff --git a/src/lisp.h b/src/lisp.h
index 77e11ccdae7..e126f575275 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -2388,6 +2388,7 @@ EXFUN (Fstring_make_unibyte, 1);
 EXFUN (Fstring_as_multibyte, 1);
 EXFUN (Fstring_as_unibyte, 1);
 EXFUN (Fstring_to_multibyte, 1);
+EXFUN (Fstring_to_unibyte, 2);
 EXFUN (Fsubstring, 3);
 extern Lisp_Object substring_both P_ ((Lisp_Object, int, int, int, int));
 EXFUN (Fnth, 2);