diff --git a/src/proto/option.pro b/src/proto/option.pro
index cd3afbf..4347d65 100644
--- a/src/proto/option.pro
+++ b/src/proto/option.pro
@@ -23,6 +23,7 @@
 char_u *check_stl_option __ARGS((char_u *s));
 int get_option_value __ARGS((char_u *name, long *numval, char_u **stringval, int opt_flags));
 int get_option_value_strict __ARGS((char_u *name, long *numval, char_u **stringval, int opt_type, void *from));
+char_u *option_iter_next __ARGS((void **option, int opt_type));
 char_u *set_option_value __ARGS((char_u *name, long number, char_u *string, int opt_flags));
 char_u *get_term_code __ARGS((char_u *tname));
 char_u *get_highlight_default __ARGS((void));