Why is RestSharp returning text/html instead of json?

I'm trying to build micro service with TMDB API.

Code:

using System.Text.Json;
using Microsoft.Extensions.Options;
using Net5Micro.Config;
using RestSharp;

namespace Net5Micro.Services
{
    public class ApiClient : IApiClient
    {
        private readonly ServiceSettings _settings;
        
        public ApiClient(IOptions<ServiceSettings> settings)
        {
            _settings = settings.Value;
        }
        
        public Movie GetById(string id)
        {
            //Init rest client
            var client = new RestClient($"{_settings.TMDBhost}/movie/{id}");
            
            //Init rest request
            var request = new RestRequest(Method.GET);
            request.RequestFormat = DataFormat.Json;

            //Add Request params
            request.AddParameter("api_key", _settings.ApiKey, ParameterType.GetOrPost);

            //Calling Api 
            var response = client.Get(request);
            
            var movies = JsonSerializer.Deserialize<Movie>(response.Content);
            
            return movies;
        }

        // Za najpopularnije
        public Movies Discover(string sort)
        {
            //Init rest client
            var client = new RestClient($"{_settings.TMDBhost}/discover/movie/");
            
            //Init rest request
            var request = new RestRequest(Method.GET);
            request.RequestFormat = DataFormat.Json;
            
            //Add Request params
            request.AddParameter("api_key", _settings.ApiKey, ParameterType.GetOrPost);
            request.AddParameter("sort_by", sort, ParameterType.GetOrPost);
            
            //Calling Api 
            var response = client.Get(request);
            
            var movies = JsonSerializer.Deserialize<Movies>(response.Content);
            
            return movies;
        }
        
        public record Movie(int id, string original_title, string poster_path);
        public record Movies(int page, Movie[] results);
    }
}

GetById() works fine, but Discover() throws an error. On debugging I discovered the value returned is text/html instead of json; I think that's the problem.

And here is the error

Exception has occurred: CLR/System.Text.Json.JsonException

An exception of type 'System.Text.Json.JsonException' occurred in System.Text.Json.dll but was not handled in user code: 'The input does not contain any JSON tokens. Expected the input to start with a valid JSON token, when isFinalBlock is true. Path: $ | LineNumber: 0 | BytePositionInLine: 0.'

Inner exceptions found, see $exception in variables window for more details.

Innermost exception
System.Text.Json.JsonReaderException : The input does not contain any JSON tokens. Expected the input to start with a valid JSON token, when isFinalBlock is true. LineNumber: 0 | BytePositionInLine: 0.

at System.Text.Json.ThrowHelper.ThrowJsonReaderException(Utf8JsonReader& json, ExceptionResource resource, Byte nextByte, ReadOnlySpan1 bytes) at System.Text.Json.Utf8JsonReader.Read() at System.Text.Json.Serialization.JsonConverter1.ReadCore(Utf8JsonReader& reader, JsonSerializerOptions options, ReadStack& state))

I'm new to .NET so I don't know how to fix it. Thanks in advance.