"Cannot coerce null to a string" in build-mix

Hi and thanks for this interesting project!

I just tried it out on two different Elixir projects, but I cannot get it work on either of them. After creating a default.nix just as described in the README and running nix-build I get the following output:

$ nix-build
error: cannot coerce null to a string, at /nix/store/nh38qpbmmrli8w3zqv00jv120chf1biz-nixos-18.09.1819.76aafbf4bf4/nixos/pkgs/development/beam-modules/build-mix.nix:32:13

I'm no Nix expert, but it seems that the issue is that name is not set when calling build-mix.nix. Does anyone have a good idea what could be wrong and how to fix it?

I'm running NixOS 18.09 on my end (although it seems to me that mix-to-nix is using a pinned version of nixpkgs).