diff --git a/src/time.c b/src/time.c
index 78e20eb..00275ef 100644
--- a/src/time.c
+++ b/src/time.c
@@ -481,6 +481,7 @@
     argv[0].vval.v_number = (varnumber_T)timer->tr_id;
     argv[1].v_type = VAR_UNKNOWN;
 
+    rettv.v_type = VAR_UNKNOWN;
     call_callback(&timer->tr_callback, -1, &rettv, 1, argv);
     clear_tv(&rettv);
 }
@@ -854,6 +855,13 @@
     callback = get_callback(&argvars[1]);
     if (callback.cb_name == NULL)
 	return;
+    if (in_vim9script() && *callback.cb_name == NUL)
+    {
+	// empty callback is not useful for a timer
+	emsg(_(e_invalid_callback_argument));
+	free_callback(&callback);
+	return;
+    }
 
     timer = create_timer(msec, repeat);
     if (timer == NULL)