diff --git a/pkgs/stdenv/booter.nix b/pkgs/stdenv/booter.nix
index b1e1e898aa3..11ca8e1440e 100644
--- a/pkgs/stdenv/booter.nix
+++ b/pkgs/stdenv/booter.nix
@@ -46,7 +46,10 @@ stageFuns: let
   # isn't already set.
   withAllowCustomOverrides = lib.lists.imap
     (index: stageFun: prevStage:
-      { allowCustomOverrides = index == 1; } # first element, 1-indexed
+      # So true by default for only the first element because one
+      # 1-indexing. Since we reverse the list, this means this is true
+      # for the final stage.
+      { allowCustomOverrides = index == 1; }
       // (stageFun prevStage))
     (lib.lists.reverseList stageFuns);