Error: "unpacker produced multiple directories"

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Hey everyone, I am trying to debug an issue with Nix, where I am trying to install a new package. I am using opam-nix to manage my dependencies automatically in Nix. However, I added a new package called timedesc and ran into the issue

error: builder for '/nix/store/2xrb8jjl70y9bph3340fhc6yp8yi5yfc-timedesc-tzdb-2.0.0.drv' failed with exit code 1;
       last 3 log lines:
       > unpacking sources
       > unpacking source archive /nix/store/7vk36zdb2sfy4x6sv7grfm87jgq414hk-timedesc-2.0.0.tar.gz
       > unpacker produced multiple directories
       For full logs, run 'nix log /nix/store/2xrb8jjl70y9bph3340fhc6yp8yi5yfc-timedesc-tzdb-2.0.0.drv'.

My nix config looks like this:

{
  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs";
    opam-nix.url = "github:tweag/opam-nix";
    flake-utils.url = "github:numtide/flake-utils";
    nixpkgs.follows = "opam-nix/nixpkgs";
  };
  outputs = { self, flake-utils, opam-nix, nixpkgs }@inputs:
    let package = "fixgen";
    in flake-utils.lib.eachDefaultSystem (system:
      let
        sourceRoot = ".";
        pkgs = nixpkgs.legacyPackages.${system};
        on = opam-nix.lib.${system};
        devPackagesQuery = {
          ocaml-lsp-server = "*";
          ocamlformat = "*";
          ocamlfind = "1.9.5";
          utop = "*";
        };
        query = devPackagesQuery // {
          ocaml-base-compiler = "*";
        };
        scope = on.buildOpamProject' { } ./. query;
        overlay = final: prev: {
          # You can add overrides here
          ${package} = prev.${package}.overrideAttrs (_: {
          });
        };
        scope' = scope.overrideScope' overlay;
        # The main package containing the executable
        main = scope'.${package};
        # Packages from devPackagesQuery
        devPackages = builtins.attrValues
          (pkgs.lib.getAttrs (builtins.attrNames devPackagesQuery) scope');
      in {
        legacyPackages = scope';

        packages.default = main;

        devShells.default = pkgs.mkShell {
          inputsFrom = [ main ];
          buildInputs = devPackages ++ [
          ];
        };
      });
}

Googling around, it seems it might have something to do with the sourceRoot=“.” parameter missing, but I am not sure where and how to add it when using something like opam-nix on top. Any pointers?