title: JSON Error in iOS
body: <p>i need to send json dictionary as parameter, i am using afnetworking. While using following code i am getting an exception:</p>
  
  <pre><code>  AFHTTPRequestOperationManager *manager = [AFHTTPRequestOperationManager manager];
      manager.responseSerializer.acceptableContentTypes = [NSSet setWithObjects:@"application/json", nil];
      manager.requestSerializer = [AFJSONRequestSerializer serializer];
      manager.responseSerializer = [AFJSONResponseSerializer serializer];
  
      NSDictionary *params = @{@"email":txt_email.text,@"password":txt_password.text,@"platform":@"iphone"};
      NSLog(@"params:%@",params);
      SBJsonWriter *jsonWriter=[[SBJsonWriter alloc]init];
  
      NSString *paramsDicJSONN = [jsonWriter stringWithObject:params];            
       NSString *str_url=[NSString stringWithFormat:@"%@do_signup",BaseURLlogin];
  
     [manager POST:str_url parameters:paramsDicJSONN success:^(AFHTTPRequestOperation *operation, id responseObject) {
          NSLog(@"offerJSON: %@", responseObject);
  </code></pre>
  
  <p>Exception:</p>
  
  <pre><code>Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** +[NSJSONSerialization dataWithJSONObject:options:error:]: Invalid top-level type in JSON write'
  </code></pre>