diff --git a/nixos/tests/usbguard.nix b/nixos/tests/usbguard.nix
index cba905db44f3..bb707bdbf702 100644
--- a/nixos/tests/usbguard.nix
+++ b/nixos/tests/usbguard.nix
@@ -22,7 +22,7 @@ import ./make-test-python.nix ({ pkgs, ... }: {
 
   testScript = ''
     # create a blank disk image for our fake USB stick
-    with open(machine.state_dir + "/usbstick.img", "wb") as stick:
+    with open(machine.state_dir / "usbstick.img", "wb") as stick:
         stick.write(b"\x00" * (1024 * 1024))
 
     # wait for machine to have started and the usbguard service to be up