diff --git a/1systems/iso.nix b/1systems/iso.nix
index 1e4f9c5..c679241 100644
--- a/1systems/iso.nix
+++ b/1systems/iso.nix
@@ -32,19 +32,24 @@ with import <stockholm/lib>;
   # hack `tee` behavior
   nixpkgs.config.packageOverrides = super: {
     irc-announce = super.callPackage <stockholm/krebs/5pkgs/simple/irc-announce> {
-      pkgs = pkgs // { coreutils = pkgs.concat "coreutils-hack" [
-        pkgs.coreutils
-        (pkgs.writeDashBin "tee" ''
-          if test "$1" = /dev/stderr; then
-            while read -r line; do
-              echo "$line"
-              echo "$line" >&2
-            done
-          else
-            ${super.coreutils}/bin/tee "$@"
-          fi
-        '')
-      ];};
+      pkgs = pkgs // {
+        coreutils = pkgs.symlinkJoin {
+          name =  "coreutils-hack";
+          paths = [
+            pkgs.coreutils
+            (pkgs.writeDashBin "tee" ''
+              if test "$1" = /dev/stderr; then
+                while read -r line; do
+                  echo "$line"
+                  echo "$line" >&2
+                done
+              else
+                ${super.coreutils}/bin/tee "$@"
+              fi
+            '')
+          ];
+        };
+      };
     };
   };
 }