diff --git a/src/proto/vim9execute.pro b/src/proto/vim9execute.pro
index eac8979..755b7dd 100644
--- a/src/proto/vim9execute.pro
+++ b/src/proto/vim9execute.pro
@@ -1,4 +1,5 @@
 /* vim9execute.c */
+void to_string_error(vartype_T vartype);
 int call_def_function(ufunc_T *ufunc, int argc_arg, typval_T *argv, partial_T *partial, typval_T *rettv);
 void ex_disassemble(exarg_T *eap);
 int tv2bool(typval_T *tv);