diff --git a/nixos/modules/virtualisation/virtualbox-host.nix b/nixos/modules/virtualisation/virtualbox-host.nix
index 3523997ce84e..6f7370181740 100644
--- a/nixos/modules/virtualisation/virtualbox-host.nix
+++ b/nixos/modules/virtualisation/virtualbox-host.nix
@@ -84,7 +84,7 @@ in
 
   config = mkIf cfg.enable (mkMerge [{
     warnings = mkIf (config.nixpkgs.config.virtualbox.enableExtensionPack or false)
-      "'nixpkgs.virtualbox.enableExtensionPack' has no effect, please use 'virtualisation.virtualbox.host.enableExtensionPack'";
+      ["'nixpkgs.virtualbox.enableExtensionPack' has no effect, please use 'virtualisation.virtualbox.host.enableExtensionPack'"];
     boot.kernelModules = [ "vboxdrv" "vboxnetadp" "vboxnetflt" ];
     boot.extraModulePackages = [ kernelModules ];
     environment.systemPackages = [ virtualbox ];