flake-info --json returns "Branch not found"

[fgaz]

When I try to run flake-info as written in the readme I get an error:

$ nix run nixos-search#flake-info -- --json nixpkgs nixos-21.05
Error: Getting nixpkgs info caused an error: GitHub returned 404 {"message":"Branch not found","documentation_url":"https://docs.github.com/rest/branches/branches#get-a-branch"}