Calling nix-env -qa results in failure on not-yet-built project

While Mix to Nix is working well for building projects, I get an error while trying to use nix-env when a project has not been built yet.

We use nix-env to list all available derivations in our internal nix-overlay, so that we can make our CI server build each derivation in turn.

Now, if I create a dummy Elixir project using mix new, setup a default.nix according to the docs, and then call nix-env -f default.nix -qa --show-trace in that directory (without calling nix-build first), I get this:

error: while evaluating 'importElixir' at /nix/store/gz1a1xq552xg69r2iqsnk3hl4skdvslb-source/default.nix:240:18, called
 from /nix/store/gz1a1xq552xg69r2iqsnk3hl4skdvslb-source/default.nix:259:19:                                          
while evaluating 'importJSON' at /nix/store/nh38qpbmmrli8w3zqv00jv120chf1biz-nixos-18.09.1819.76aafbf4bf4/nixos/lib/tri
vial.nix:237:16, called from /nix/store/gz1a1xq552xg69r2iqsnk3hl4skdvslb-source/default.nix:240:24:                   
cannot read '/nix/store/yhhb0ih007drznbwd76h8khywbp8pyk1-elixir-term.json', since path '/nix/store/4jv30jsybmwlw9pjay3c
m2a15dxkj1ki-elixir-term.json.drv' is not valid, at /nix/store/nh38qpbmmrli8w3zqv00jv120chf1biz-nixos-18.09.1819.76aafb
f4bf4/nixos/lib/trivial.nix:238:24

It looks like some kind of bootstrapping problem to me, since the drv-path doesn't exist yet when we try to import it. I'm not fully understanding what to do about it, however. Any pointers? Thank you!