error 超时处理是返回为空的为什么 

    ASI
NSURL *url = [NSURL URLWithString:_URL];
ASIHTTPRequest *request = [[ASIHTTPRequest alloc] initWithURL:url];
[request setDelegate:self];
[request appendPostData:[param dataUsingEncoding:NSUTF8StringEncoding]];
[request setRequestMethod:@"PUT"];
[request setTimeOutSeconds:30];//设置超时 

[request startSynchronous];

NSError *error = [request error];//error 超时处理是返回为空的为什么
if (error)
{
DLog("%@",error);
NSException * tException=[NSException exceptionWithName:@"网络连接失败" reason:[error domain] userInfo:[error userInfo]];
@throw tException;
}