How can I create an index from a HashSet in Marten while working on a C# .NET Core event-sourcing project?

Question

I am current working in a project using event sourcing with Marten.

And happens that when trying to create a index it doesn't accept HashSet from the projection returning an error as shown here:

CREATE INDEX mt_doc_taskprojection_idx_staff 
ON public.mt_doc_taskprojection USING btree ((CAST(data ->> 'Staff' as uuid[])));
      )' raised.

public static class DependencyInjection
{
    public static IServiceCollection AddPlannerDB(this IServiceCollection services)
    {
        ConfigurationBuilder builder = new ();
        builder.AddUserSecrets<Startup>();
        IConfigurationRoot config = builder.Build();
        
        services
            .AddMartenStore<IPlannerStore>(options =>
            {
                options.Connection(config["bitCube-planner"]!);
                options.Projections
                    .SelfAggregate<WorkspaceProjection>(ProjectionLifecycle.Inline)
                    .Index(projection => projection.Id)
                    .Index(projection => projection.Name);
                options.Projections
                    .SelfAggregate<TaskGroupProjection>(ProjectionLifecycle.Inline)
                    .Index(projection => projection.Id)
                    .Index(projection => projection.Name)
                    .Index(projection => projection.WorkspaceId);
                options.Projections
                    .SelfAggregate<TaskProjection>(ProjectionLifecycle.Inline)
                    .Index(projection => projection.Id)
                    .Index(projection => projection.Title)
                    .Index(projection => projection.WorkspaceId)
                    .Index(projection => projection.Staff.Select(id => id));
            });
        
        services.AddTransient<IWorkspaceRepository, WorkspaceRepository>();
        services.AddTransient<ITaskGroupRepository, TaskGroupRepository>();
        services.AddTransient<ITaskRepository, TaskRepository>();
        return services;
    }
}

And here is the line where staff is passed which is a HashSet<Guid>

.Index(projection => projection.Staff.Select(id => id));

Question is how can I convert it to a list or create a index from a HashSet?

I have tried this solution, but not sure if is the best approach

List<Guid> staffList = new TaskProjection().Staff.Select(id => id).ToList();
options.Projections
    .SelfAggregate<TaskProjection>(ProjectionLifecycle.Inline)
    .Index(projection => projection.Id)
    .Index(projection => projection.Title)
    .Index(projection => projection.WorkspaceId)
    .Index(projection => staffList);

It creates the index