Build with nix fails after update to v1.1

I just updated my app to IHP v1.1, everything went smoothly, I can run the development server without any issue. However, when I try to run nix-build or nix build, I get errors (I'm not sure which is the correct command at the moment, with flakes it should be nix build, but the documentation still says nix-build, with hyphen). The error message I get with nix-build:

error:
       … while calling the 'derivationStrict' builtin

         at //builtin/derivation.nix:9:12: (source not available)

       … while evaluating derivation 'app'
         whose name attribute is located at /nix/store/4jh9cp71v65rxlx9yk3lgp4g1x0nw7i5-source/pkgs/stdenv/generic/make-derivation.nix:302:7

       … while evaluating attribute 'buildInputs' of derivation 'app'

         at /nix/store/4jh9cp71v65rxlx9yk3lgp4g1x0nw7i5-source/pkgs/stdenv/generic/make-derivation.nix:345:7:

          344|       depsHostHost                = lib.elemAt (lib.elemAt dependencies 1) 0;
          345|       buildInputs                 = lib.elemAt (lib.elemAt dependencies 1) 1;
             |       ^
          346|       depsTargetTarget            = lib.elemAt (lib.elemAt dependencies 2) 0;

       error: cannot compare a set with a string with context

with nix build I got some error:

error:
       … while evaluating the attribute 'optionalValue.value'

         at /nix/store/3i5iasvc4035gna928p7iwnmv8nify1k-source/lib/modules.nix:806:5:

          805|
          806|     optionalValue =
             |     ^
          807|       if isDefined then { value = mergedValue; }

       … while evaluating a branch condition

         at /nix/store/3i5iasvc4035gna928p7iwnmv8nify1k-source/lib/modules.nix:807:7:

          806|     optionalValue =
          807|       if isDefined then { value = mergedValue; }
             |       ^
          808|       else {};

       (stack trace truncated; use '--show-trace' to show the full trace)

       error: cannot look up '<nixpkgs>' in pure evaluation mode (use '--impure' to override)

       at «none»:0: (source not available)

I ended up using nix build --impure, but I'd prefer a pure solution