如图所示 为什么总是返回 invalid parameter(s)

NSString *urlString = [NSString stringWithFormat:@"https://aip.baidubce.com/rpc/2.0/nlp/v2/simnet?charset=UTF-8&access_token=%@",accessTokenString];

    AFHTTPSessionManager *sessionManager = [AFHTTPSessionManager manager];

    NSDictionary *paraDict = @{@"text_1":@"How are you",@"text_2":@"How is going",@"model":@"CNN"};

    NSData *data = [NSJSONSerialization dataWithJSONObject:paraDict options:NSJSONWritingPrettyPrinted error:nil];

    NSString *utf8String = [[NSString alloc]initWithData:data encoding:NSUTF8StringEncoding];

    [sessionManager POST:urlString parameters:utf8String progress:nil success:^(NSURLSessionDataTask * _Nonnull task, id  _Nullable responseObject) {

        if (responseObject) {

                              }

    } failure:^(NSURLSessionDataTask * _Nullable task, NSError * _Nonnull error) {

    }];