diff --git a/src/proto/userfunc.pro b/src/proto/userfunc.pro
index ce5d257..32dac66 100644
--- a/src/proto/userfunc.pro
+++ b/src/proto/userfunc.pro
@@ -39,7 +39,7 @@
 varnumber_T call_callback_retnr(callback_T *callback, int argcount, typval_T *argvars);
 void user_func_error(funcerror_T error, char_u *name, int found_var);
 int call_func(char_u *funcname, int len, typval_T *rettv, int argcount_in, typval_T *argvars_in, funcexe_T *funcexe);
-int call_simple_func(char_u *funcname, int len, typval_T *rettv);
+int call_simple_func(char_u *funcname, size_t len, typval_T *rettv);
 char_u *printable_func_name(ufunc_T *fp);
 char_u *trans_function_name(char_u **pp, int *is_global, int skip, int flags);
 char_u *trans_function_name_ext(char_u **pp, int *is_global, int skip, int flags, funcdict_T *fdp, partial_T **partial, type_T **type, ufunc_T **ufunc);