diff --git a/src/evalfunc.c b/src/evalfunc.c
index c092421..a5e4b20 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -7534,7 +7534,7 @@
  * Currently only valid for object/container types.
  * Return empty string if not an object.
  */
-    void
+    static void
 f_id(typval_T *argvars, typval_T *rettv)
 {
     char    numbuf[NUMBUFLEN];