diff --git a/src/errors.h b/src/errors.h
index 34c47ca..bc6258c 100644
--- a/src/errors.h
+++ b/src/errors.h
@@ -274,4 +274,6 @@
 	INIT(= N_("E1124: \"%s\" cannot be used in legacy Vim script"));
 EXTERN char e_final_requires_a_value[]
 	INIT(= N_("E1125: Final requires a value"));
+EXTERN char e_cannot_use_let_in_vim9_script[]
+	INIT(= N_("E1126: Cannot use :let in Vim9 script"));
 #endif