diff --git a/src/proto/option.pro b/src/proto/option.pro
index 49e1d9a..4a49d89 100644
--- a/src/proto/option.pro
+++ b/src/proto/option.pro
@@ -17,6 +17,7 @@
 void free_string_option __ARGS((char_u *p));
 void clear_string_option __ARGS((char_u **pp));
 void set_term_option_alloced __ARGS((char_u **p));
+int was_set_insecurely __ARGS((char_u *opt));
 void set_string_option_direct __ARGS((char_u *name, int opt_idx, char_u *val, int opt_flags));
 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));