diff --git a/nixos/tests/isso.nix b/nixos/tests/isso.nix
index f4560ba3e635..575e1c52eccf 100644
--- a/nixos/tests/isso.nix
+++ b/nixos/tests/isso.nix
@@ -22,7 +22,7 @@ import ./make-test-python.nix ({ pkgs, ... }: {
   ''
     machine.wait_for_unit("isso.service")
 
-    machine.wait_for_open_port(port)
+    machine.wait_for_open_port(${toString port})
 
     machine.succeed("curl --fail http://localhost:${toString port}/?uri")
     machine.succeed("curl --fail http://localhost:${toString port}/js/embed.min.js")