diff --git a/nixos/modules/config/users-groups.nix b/nixos/modules/config/users-groups.nix
index 873b6fb424de..5de81a773424 100644
--- a/nixos/modules/config/users-groups.nix
+++ b/nixos/modules/config/users-groups.nix
@@ -509,7 +509,7 @@ in {
         message = "uids and gids must be unique!";
       }
       { assertion = cfg.mutableUsers || (nonUidUsers == {});
-        message = "When mutableUsers is false, no uid can be null";
+        message = "When mutableUsers is false, no uid can be null: ${toString (attrNames nonUidUsers)}";
       }
       { assertion = cfg.mutableUsers || (nonGidGroups == {});
         message = "When mutableUsers is false, no gid can be null";