Why always crash in xmlFreeDoc(xmlDoc_)

- (void)dealloc {
    if (xmlDoc_ != NULL) {
        // release the strings cache
        //
        // since it's a CF object, were anyone to use this in a GC environment,
        // this would need to be released in a finalize method, too
        if (xmlDoc_->_private != NULL) {
            CFRelease(xmlDoc_->_private);
        }
        
        xmlFreeDoc(xmlDoc_);
    }
}