iOS POST JSON Echo Disallowed Key Characters

Question

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 tring to post this dictionary.The groupArr consist of Addressbook Contacts.But its is giving me Echo message form server Echo Disallowed Key CharatcersI am not getting y it is throwing this error