diff --git a/pkgs/stdenv/custom/default.nix b/pkgs/stdenv/custom/default.nix
index c8f1bc66a06c..4520f0c9905a 100644
--- a/pkgs/stdenv/custom/default.nix
+++ b/pkgs/stdenv/custom/default.nix
@@ -31,7 +31,10 @@ bootStages
     stdenv =
       assert vanillaPackages.stdenv.hostPlatform == localSystem;
       assert vanillaPackages.stdenv.targetPlatform == localSystem;
-      config.replaceStdenv { pkgs = vanillaPackages; };
+      let
+        fn = config.replaceStdenv or null;
+      in
+      if fn == null then vanillaPackages.stdenv else fn { pkgs = vanillaPackages; };
   })
 
 ]