diff --git a/src/proto/eval.pro b/src/proto/eval.pro
index df59383..47be0b9 100644
--- a/src/proto/eval.pro
+++ b/src/proto/eval.pro
@@ -74,5 +74,5 @@
 void ex_execute(exarg_T *eap);
 char_u *find_option_end(char_u **arg, int *scope);
 void last_set_msg(sctx_T script_ctx);
-char_u *do_string_sub(char_u *str, size_t str_len, char_u *pat, char_u *sub, typval_T *expr, char_u *flags, size_t *ret_len);
+char_u *do_string_sub(char_u *str, size_t len, char_u *pat, char_u *sub, typval_T *expr, char_u *flags, size_t *ret_len);
 /* vim: set ft=c : */