Vsftp fails to start

I am trying to enable vsftp on my NixOS unstable system. I have the following in configuration.nix:

  services.vsftpd = {
    enable = true;
    anonymousUser = true;
    anonymousUserNoPassword = true;
    localUsers = true;
    writeEnable = true;

The service is not starting. The journal has the following messages:

Starting Vsftpd Server...
Started Vsftpd Server.
vsftpd.service: Main process exited, code=exited, status=2/INVALIDARGUMENT
vsftpd.service: Failed with result 'exit-code'.

Any clues?