Not able to infer fields from extended core User schema

export const auth = betterAuth({
  database: pgPool,
  user: {
    additionalFields: {
      firstName: {
        type: "string",
        required: false,
      },
      lastName: {
        type: "string",
        required: false,
      },
    },... 

export const authClient = createAuthClient({
  baseURL: "http://localhost:3000",
  plugins: [inferAdditionalFields<typeof auth>(), anonymousClient()],
});

Type error: Property 'isAnonymous' does not exist on type '{ agencies: {} | null; id: string; name: string; emailVerified: boolean; email: string; createdAt: Date; updatedAt: Date; image?: string | null | undefined; }'.

  17 |   });
  18 |
> 19 |   const isAuthenticatedUser = session?.user && !session.user.isAnonymous;