diff --git a/src/errors.h b/src/errors.h
index 990fb86..e6d7ab2 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -351,3 +351,5 @@
 	INIT(= N_("E1156: Cannot change the argument list recursively"));
 EXTERN char e_missing_return_type[]
 	INIT(= N_("E1157: Missing return type"));
+EXTERN char e_cannot_use_flatten_in_vim9_script[]
+	INIT(= N_("E1158: Cannot use flatten() in Vim9 script"));