The current default value of listenAddress = null blows up: $ nixos-rebuild build error: cannot coerce null to a string, at .../nixpkgs/nixos/modules/services/monitoring/prometheus/alertmanager.nix:97:16 With listenAddress = "" we use the same default as upstream and there is no blow up :-)
*** NixOS *** NixOS is a Linux distribution based on the purely functional package management system Nix. More information can be found at http://nixos.org/nixos and in the manual in doc/manual.