Error running `nix-update --flake --use-update-script`

I have a personal flake repo that containing some packages. I would like to use passthru.userScript = nix-update-script {} in some packages to test thing out. I can successfully run nix-update package1 --flake in the root of the flake repo. However when I run nix-update package1 --flake --use-update-script it throws error about error: path '/home/user/path/of/flake/repo/default.nix' does not exist I cannot find any documentation about that in the repo README.md usage. My flake only have flake....