Let say that I have an overlay folder in my configuration and a file `default.nix` containing : 
```
with builtins;
 let path = ./.; in {
       nixpkgs.overlays = [(map (n: import (path + ("/" + n)))
          (filter (n: match ".*\\.nix" n != null ||
                      pathExists (path + ("/" + n + "/default.nix")))
                  (attrNames (readDir path)))
       )];
}
```
Why would `nixos-rebuild switch` tell me `The option value `nixpkgs.overlays.[definition 1-entry 1]' in path/to/overlays/default.nix' is not of type `nixpkgs overlay'` ?