I can't initialize a session

import { createAsync, query } from "@solidjs/router";
import { useSession } from "vinxi/http";

type SessionData = {
  theme: "light" | "dark";
};

const getTheme = query(async () => {
  "use server";
  const session = await useSession<SessionData>({
    password: process.env.SESSION_SECRET as string,
    name: "theme",
  });

  if (!session.data.theme) {
    await session.update({
      theme: "light",
    });
  }

  return session.data.theme;
}, "getTheme");

export function ThemeToggle() {
  const theme = createAsync(() => getTheme());

  return <p>theme: {theme()}</p;
}

Cannot set headers after they are sent to the client