diff --git a/src/proto/evalfunc.pro b/src/proto/evalfunc.pro
index bcb4ef5..4e9ad5b 100644
--- a/src/proto/evalfunc.pro
+++ b/src/proto/evalfunc.pro
@@ -20,6 +20,7 @@
 void execute_common(typval_T *argvars, typval_T *rettv, int arg_off);
 void f_exists(typval_T *argvars, typval_T *rettv);
 void f_has(typval_T *argvars, typval_T *rettv);
+void f_len(typval_T *argvars, typval_T *rettv);
 int dynamic_feature(char_u *feature);
 void mzscheme_call_vim(char_u *name, typval_T *args, typval_T *rettv);
 void range_list_materialize(list_T *list);