diff --git a/src/proto/vim9compile.pro b/src/proto/vim9compile.pro
index 6bf3eb3..4108c04 100644
--- a/src/proto/vim9compile.pro
+++ b/src/proto/vim9compile.pro
@@ -6,7 +6,7 @@
 int check_typval_type(type_T *expected, typval_T *actual_tv);
 int check_type(type_T *expected, type_T *actual, int give_msg);
 int check_compare_types(exptype_T type, typval_T *tv1, typval_T *tv2);
-char_u *skip_type(char_u *start);
+char_u *skip_type(char_u *start, int optional);
 type_T *parse_type(char_u **arg, garray_T *type_gap);
 char *vartype_name(vartype_T type);
 char *type_name(type_T *type, char **tofree);