diff --git a/nixos.go b/nixos.go
index 302638c..7b06079 100644
--- a/nixos.go
+++ b/nixos.go
@@ -3,7 +3,7 @@ package main
 import "path/filepath"
 
 func mkNixOSImage(configFname, cacheDir, vmID string) (string, error) {
-	outputFname := filepath.Join(cacheDir, "nixos", vmID+".qcow2")
+	outputFname := filepath.Join(cacheDir, "nixos", vmID)
 	err := run("nix-shell", "-p", "nixos-generators", "--run", "nixos-generate -f openstack -o "+outputFname+" -c "+configFname)
 	if err != nil {
 		return "", err