diff --git a/nixos/modules/services/misc/nix-daemon.nix b/nixos/modules/services/misc/nix-daemon.nix
index 4b199aa22b50..ba3ea4c47ac1 100644
--- a/nixos/modules/services/misc/nix-daemon.nix
+++ b/nixos/modules/services/misc/nix-daemon.nix
@@ -228,13 +228,16 @@ in
               '';
             };
             protocol = mkOption {
-              type = types.enum [ "ssh" "ssh-ng" ];
+              type = types.enum [ null "ssh" "ssh-ng" ];
               default = "ssh";
               example = "ssh-ng";
               description = lib.mdDoc ''
                 The protocol used for communicating with the build machine.
                 Use `ssh-ng` if your remote builder and your
                 local Nix version support that improved protocol.
+
+                Use `null` when trying to change the special localhost builder
+                without a protocol which is for example used by hydra.
               '';
             };
             system = mkOption {
@@ -680,7 +683,7 @@ in
         concatMapStrings
           (machine:
             (concatStringsSep " " ([
-              "${machine.protocol}://${optionalString (machine.sshUser != null) "${machine.sshUser}@"}${machine.hostName}"
+              "${optionalString (machine.protocol != null) "${machine.protocol}://"}${optionalString (machine.sshUser != null) "${machine.sshUser}@"}${machine.hostName}"
               (if machine.system != null then machine.system else if machine.systems != [ ] then concatStringsSep "," machine.systems else "-")
               (if machine.sshKey != null then machine.sshKey else "-")
               (toString machine.maxJobs)