[Nix-dev] imported archive lacks a signature?

[Jeffrey David Johnson]

What does this error mean?

$ nix-copy-closure --to jefdaj at server $(type -tP labwiki)
copying 166 missing paths (1820.22 MiB) to ‘jefdaj at server’...
exporting path ‘/nix/store/cgddwzz9hkdgprvbymphv8yprc66zxk7-ghc-7.10.1’
exporting path ‘/nix/store/0j60852jsskjl2x3mv0a1ssrkb18hymz-haskell-old-locale-1.0.0.7’
exporting path ‘/nix/store/21k662hg1jics83kh9x2r3cfp9fd4cll-haskell-network-2.6.1.0’
error: imported archive of ‘/nix/store/cgddwzz9hkdgprvbymphv8yprc66zxk7-ghc-7.10.1’ lacks a signature

I can't find anything about signatures in https://nixos.org/nixos/manual/.

Thanks
Jeff