if let value: NSString = self.filterSender { // Casted to NSString
    let sender = value.cStringUsingEncoding(NSUTF8StringEncoding) // NSString.cStringUsingEncoding()
    asl_set_query(query, ASL_KEY_SENDER, sender, UInt32(ASL_QUERY_OP_EQUAL | ASL_QUERY_OP_SUBSTRING))
}