Can't do hash update on Go module in a flake

[nyarly]

I have a flake with an output like myprogram = pkgs.buildGoModule {..., and nix-update --flake --version=skip myprogram errors with:

error: value is null while a set was expected

       at «string»:22:5:

           21|    else
           22|     sanitizePosition (builtins.unsafeGetAttrPos "src" pkg);
             |     ^
           23| in {

(as an alternative, I tried extracting to a bare expression that I could callPackage, but since the package was then the file root, I couldn't figure out how to get nix-update to address it.)