IOS POST JSON Echo Forbidden Key Characters

NSMutableDictionary *dictionary=[[NSMutableDictionary alloc] init];

[dictionary setObject:self.uniqueKey forKey:@"unique"];


[dictionary setObject:groupArr forKey:@"addressbook"];



if ([NSJSONSerialization isValidJSONObject:dictionary]){

    NSURL *uploadPath=[NSURL URLWithString:@"URL"];

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

   }

      

        
        
        
      

    

I am trying to post this dictionary. groupArr

consists of Addressbook Contacts.But it is given to me by the ping form server Echo Disallowed Key Charatcers

I am not getting y, it is throwing this error