diff --git a/callUnitRoot.nix b/callUnitRoot.nix
index 64f945b..6ee290d 100644
--- a/callUnitRoot.nix
+++ b/callUnitRoot.nix
@@ -7,9 +7,9 @@
 let
   shards = lib.attrNames (builtins.readDir root);
   namesForShard = shard: lib.mapAttrs'
-    (name: _: { inherit name; value = "${root}/${shard}/${name}"; })
-    (builtins.readDir "${root}/${shard}");
+    (name: _: { inherit name; value = root + "/${shard}/${name}"; })
+    (builtins.readDir (root + "/${shard}"));
   namesToPath = lib.foldl' lib.recursiveUpdate { } (map namesForShard shards);
-  units = lib.mapAttrs (_: path: pkgs.callPackage "${path}/${packageFun}" { }) namesToPath;
+  units = lib.mapAttrs (_: path: pkgs.callPackage (path + "/${packageFun}") { }) namesToPath;
 in
 units