diff --git a/tests/nix-copy-closure.nix b/tests/nix-copy-closure.nix
index 0bf5b42d8..be0a4a683 100644
--- a/tests/nix-copy-closure.nix
+++ b/tests/nix-copy-closure.nix
@@ -2,7 +2,7 @@
 
 { nixpkgs, system, nix }:
 
-with import (nixpkgs + /nixos/lib/testing.nix) { inherit system; };
+with import (nixpkgs + "/nixos/lib/testing.nix") { inherit system; };
 
 makeTest (let pkgA = pkgs.cowsay; pkgB = pkgs.wget; pkgC = pkgs.hello; in {