How to configure zramSwap.writebackDevice

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1

I would like to configure zramSwap.writebackDevice but both versions are rejected by NixOS. It builds without error but is stuck on boot until the timeout of 1:30min is over:

  zramSwap = {
    enable = true;
    memoryPercent = 20;
    # writebackDevice = "/var/zramSwap-writeback"; # TODO
    writebackDevice = "/dev/nvme0n1p3/zramswap"; # TODO
  };

I don’t understand it because defining device in swapDevices like this works:

  swapDevices = [
    {
      device = "/var/swap";
      size = 8 * 1024; # GiB
    }
  ];