diff --git a/nixos/modules/services/x11/window-managers/xmonad.nix b/nixos/modules/services/x11/window-managers/xmonad.nix
index fe8ed3812511..1ad0bb9ce677 100644
--- a/nixos/modules/services/x11/window-managers/xmonad.nix
+++ b/nixos/modules/services/x11/window-managers/xmonad.nix
@@ -43,6 +43,7 @@ in {
       haskellPackages = mkOption {
         default = pkgs.haskellPackages;
         defaultText = "pkgs.haskellPackages";
+        type = types.package;
         example = literalExample "pkgs.haskell.packages.ghc784";
         description = ''
           haskellPackages used to build Xmonad and other packages.