diff --git a/src/libexpr/nix.sdf b/src/libexpr/nix.sdf
index 8623d1d14..2b99cf09a 100644
--- a/src/libexpr/nix.sdf
+++ b/src/libexpr/nix.sdf
@@ -1,3 +1,6 @@
+%% Note: this SDF grammar is no longer used in the Nix expression
+%% parser and may not be up to date.
+
 definition