error:
       … while evaluating the attribute 'optionalValue.value'
         at «https://releases.nixos.org/nixos/26.05/nixos-26.05.6815.531670d871c0/nixexprs.tar.xz?narHash=sha256-rBYNiK4XmVxmJaK40DRnmoPUpCeRXJ4DJdQ/DW2LQRU%3D»/lib/modules.nix:1330:5:
         1329|
         1330|     optionalValue = if isDefined then { value = mergedValue; } else { };
             |     ^
         1331|   };

       … while evaluating a branch condition
         at «https://releases.nixos.org/nixos/26.05/nixos-26.05.6815.531670d871c0/nixexprs.tar.xz?narHash=sha256-rBYNiK4XmVxmJaK40DRnmoPUpCeRXJ4DJdQ/DW2LQRU%3D»/lib/modules.nix:1330:21:
         1329|
         1330|     optionalValue = if isDefined then { value = mergedValue; } else { };
             |                     ^
         1331|   };

       … while evaluating definitions from `/nix/store/lrxvs4l9bqbgjfz7w9bkfg5zqq442jxc-source/modules/transposition.nix':

       … while evaluating definitions from `/nix/store/n7wwdy88q4iikkf3wy25bgadrd6pm8sq-source/checks/flake-module.nix, via option perSystem':

       … while evaluating definitions from `/nix/store/n7wwdy88q4iikkf3wy25bgadrd6pm8sq-source/flakeModules/clan.nix':

       … while evaluating definitions from `/nix/store/n7wwdy88q4iikkf3wy25bgadrd6pm8sq-source/modules/clan/module.nix':

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error:
       nix-darwin now uses release branches that correspond to Nixpkgs releases.
       The nix-darwin and Nixpkgs branches in use must match, but you are currently
       using nix-darwin 26.11 with Nixpkgs 26.05.

       On macOS, you should use either the `nixpkgs-unstable` or
       `nixpkgs-YY.MM-darwin` branches of Nixpkgs. These correspond to the
       `master` and `nix-darwin-YY.MM` branches of nix-darwin, respectively. Check
       <https://status.nixos.org/> for the currently supported Nixpkgs releases.

       If you’re using flakes, make sure your inputs look like this:

           inputs = {
             nixpkgs.url = "github:NixOS/nixpkgs/NIXPKGS-BRANCH";
             nix-darwin.url = "github:nix-darwin/nix-darwin/NIX-DARWIN-BRANCH";
             nix-darwin.inputs.nixpkgs.follows = "nixpkgs";
             # …
           };

       If you’re using channels, you can check your current channels with:

           $ sudo nix-channel --list
           nixpkgs https://nixos.org/channels/NIXPKGS-BRANCH
           darwin https://github.com/nix-darwin/nix-darwin/archive/NIX-DARWIN-BRANCH.tar.gz
           …
           $ nix-channel --list
           …

       If `darwin` or `nixpkgs` are present in `nix-channel --list` (without
       `sudo`), you should delete them with `nix-channel --remove NAME`. These can
       contribute to version mismatch problems.

       You can then fix your channels like this:

           $ sudo nix-channel --add https://nixos.org/channels/NIXPKGS-BRANCH nixpkgs
           $ sudo nix-channel --add https://github.com/nix-darwin/nix-darwin/archive/NIX-DARWIN-BRANCH.tar.gz darwin
           $ sudo nix-channel --update

       After that, activating your system again should work correctly. If it
       doesn’t, please open an issue at
       <https://github.com/nix-darwin/nix-darwin/issues/new> and include as much
       information as possible.