Nixpkgs.config failed assertion 

Hey, just wondering how to resolve this issue now with latest in unstable introduced from https://github.com/NixOS/nixpkgs/commit/eb8ce7930d14dafcc7eff56c2f9efca6a3b2f622. 

```
Failed assertions:
       - Your system configures nixpkgs with an externally created instance.
       `nixpkgs.config` options should be passed when creating the instance instead.
```
