Can't fetch from cachix when declared in my haskell overlay

[teto]

So when I define hie in my haskell overlay https://github.com/teto/home/blob/722ca5cbce0fcc87d769af813f1e3397f4ba4293/config/nixpkgs/overlays/haskell.nix#L98 (which seemed the logical thing to do as it should depend from ghc compiler), I can't use cachix. If I move the declaration to a top-level attribute, then cachix is used.