DSharp plus, how grant a role to the custom user

Question

I couldn't give a role to a custom user. DSharp use only a Member on a GrantRoleAsync method

            var mainrole = ctx.Guild.GetRole(123456789);
            var subrole = ctx.Guild.GetRole(123456789);
            if (ctx.Member.Roles.Contains(mainrole))
            {
                await ctx.Member.GrantRoleAsync(subrole);
                var message = new DiscordEmbedBuilder()
                {
                    Color = DiscordColor.Green,
                    Title = "Role have been gave"
                };
                await ctx.Channel.SendMessageAsync(message);