Problems installing mercurial from nixpkgs-unstable

With the recent few updates to the nixpkgs-unstable,
when trying to install mercurial I get the error:

$ nix-env -iA nixpkgs.mercurial
installing 'mercurial-5.6'
nix-env: src/libexpr/attr-set.hh:54: void nix::Bindings::push_back(const nix::Attr&): Assertion `size_ < capacity_' failed.
Aborted

This is a KDE Neon system using nix packages. I get the same error even when I removed /nix and all the ~/.nix directories and reinstall nix from scratch.

If also fails when i remove my ~/.config/nixpkgs directory so none of my own configuration is seen.

Everything else in my configuration seems to work fine,
although at the time the mercurial package failed, the emacsGit from the emacs-overlay package also failed with an error about not being able to move /source under /nix/store due to a privilege violation. But that error went away after the last update.

When I use the same basic user setup I don’t have problems installing mercural on my nixos-unstable and nixos-20.09 systems.

Nobody seems to have the same issue. I eventually patched around the issue with:

  myMercurial = super.mercurial.overrideAttrs (old: {
    meta.version = null;
  });

But why the import of version to meta was an issue I don’t understand. I also don’t understand why the maintainer feels this is necessary to do.