diff --git a/pkgs/stdenv/generic/make-derivation.nix b/pkgs/stdenv/generic/make-derivation.nix
index 12a6dffb426f..f7c935168852 100644
--- a/pkgs/stdenv/generic/make-derivation.nix
+++ b/pkgs/stdenv/generic/make-derivation.nix
@@ -806,9 +806,7 @@ let
       );
 
     let
-      env' = env // {
-        ${if meta ? mainProgram then "NIX_MAIN_PROGRAM" else null} = meta.mainProgram;
-      };
+      env' = if meta ? mainProgram then env // { NIX_MAIN_PROGRAM = meta.mainProgram; } else env;
 
       derivationArg = makeDerivationArgument (
         removeAttrs attrs [