these derivations will be built:
  /nix/store/15dgafgaafrzj987m0ghvf3y56484nfy-neomutt-20171027.drv
these paths will be fetched (0.08 MiB download, 0.40 MiB unpacked):
  /nix/store/aawp98738v7dy4drl540gy3jsd6lk90n-nullmailer-2.1
error: some outputs of ‘/nix/store/15dgafgaafrzj987m0ghvf3y56484nfy-neomutt-20171027.drv’ are not valid, so checking is not possible