GPO链接，值不在预期范围内

Question

有一个将GPO链接到OU的函数：

using Microsoft.GroupPolicy;

private void LinkTheGPO()
{
    GPDomain domain = new GPDomain("city.dom");

    string sOU = "LDAP://city/ou=room,ou=house,DC=city,DC=dom";

    Som som = domain.GetSom(sOU); //ERROR AT THIS LINE

    som.LinkGpo(-1, gpo);
}

它会抛出此错误：

Value does not fall within the expected range

我的OU结构是：

City  
|  
|___House  
     |  
     |____Room