diff --git a/pkgs/applications/virtualization/virtualbox/guest-additions/default.nix b/pkgs/applications/virtualization/virtualbox/guest-additions/default.nix
index 5661ff4e0848..dbbf03daf408 100644
--- a/pkgs/applications/virtualization/virtualbox/guest-additions/default.nix
+++ b/pkgs/applications/virtualization/virtualbox/guest-additions/default.nix
@@ -76,12 +76,17 @@ stdenv.mkDerivation {
   src = "${virtualBoxNixGuestAdditionsBuilder}/VBoxGuestAdditions-${platform}.tar.bz2";
   sourceRoot = ".";
 
-  KERN_DIR = "${kernel.dev}/lib/modules/${kernel.modDirVersion}/build";
-  KERN_INCL = "${kernel.dev}/lib/modules/${kernel.modDirVersion}/source/include";
-
   hardeningDisable = [ "pic" ];
 
-  env.NIX_CFLAGS_COMPILE = "-Wno-error=incompatible-pointer-types -Wno-error=implicit-function-declaration";
+  env = {
+    NIX_CFLAGS_COMPILE = toString [
+      "-Wno-error=incompatible-pointer-types"
+      "-Wno-error=implicit-function-declaration"
+    ];
+
+    KERN_DIR = "${kernel.dev}/lib/modules/${kernel.modDirVersion}/build";
+    KERN_INCL = "${kernel.dev}/lib/modules/${kernel.modDirVersion}/source/include";
+  };
 
   nativeBuildInputs = [
     patchelf