How to solve Newtonsoft.Json Error: "Can not convert Object to String" when trying to query JSON with Linq?

Im writing a function with a get-request to get content from a Confluence page via REST API and the result is a JSON. Now I want to parse this JSON by using JObject.parse() to get a value by its property name and return the value.

using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Azure.WebJobs;
using Microsoft.Azure.WebJobs.Extensions.Http;
using Microsoft.Extensions.Logging;
using Newtonsoft.Json;
using System;
using System.Net.Http;
using System.Net.Http.Headers;
using System.Text;
using System.Threading.Tasks;
using Microsoft.Extensions.Configuration;
using Newtonsoft.Json.Linq;

namespace Azure_Functions
{
    public static class getContent
    {
        private static HttpClient httpClient = new HttpClient();
        [FunctionName("getContent")]
        public static async Task<IActionResult> getContentByID(
            [HttpTrigger(AuthorizationLevel.Anonymous, "get", "post", Route = null)] HttpRequest req,
              ILogger log, ExecutionContext context)
        {
            //Set up Configuration Builder
            var confBuild = new ConfigurationBuilder()
                .SetBasePath(context.FunctionAppDirectory)
                .AddJsonFile("local.settings.json")
                .AddEnvironmentVariables()
                .Build();

            //Basic Authentication
            var user = confBuild["ConfluenceUser"];
            var api = confBuild["ConfluenceAPI"];
            var domain = confBuild["ConfluenceDomain"];
            httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Basic",
              Convert.ToBase64String(
                  Encoding.ASCII.GetBytes(
                      string.Format("{0}:{1}", user, api))));

            string htmlbody;

            //Paste ID from Confluence page
            string pageID = "<page-id>";

            //Get content from page
            using (HttpResponseMessage response = await httpClient.GetAsync(
                        $"https://{domain}/wiki/rest/api/content/{pageID}?expand=body.storage"))
            {
                response.EnsureSuccessStatusCode();
                string responseBody = await response.Content.ReadAsStringAsync();           

                JObject body = JObject.Parse(responseBody);
                htmlbody = (string)body["body"]["storage"];
            }
            return new OkObjectResult(htmlbody);
        }

    }
}

I get the error message "Newtonsoft.Json: Can not convert Object to String.". I know that the mistake lies somewhere here:

             string htmlbody;

            //Paste ID from Confluence page
            string pageID = "<page-id>";

            //Get content from page
            using (HttpResponseMessage response = await httpClient.GetAsync(
                        $"https://{domain}/wiki/rest/api/content/{pageID}?expand=body.storage"))
            {
                response.EnsureSuccessStatusCode();
                string responseBody = await response.Content.ReadAsStringAsync();           

                JObject body = JObject.Parse(responseBody);
                htmlbody = (string)body["body"]["storage"];

            }
            return new OkObjectResult(htmlbody);

Does someone know how I can solve this error?