No package named coq-stdpp-unstable found

Following the README instructions, I can do opam install coq-stdpp but when I try to run opam install coq-stdpp-unstable I get [ERROR] No package named coq-stdpp-unstable found. I was able to build and install coq-stdpp-unstable running ./make-package stdpp_unstable and ./make-package stdpp_unstable install.