Sharepoint 2010 将自定义主页应用于所有SP基础站点

我已经创建了一个自定义母版页，它被部署为一个功能。我使用SP基金会而不是SP服务器。

我已激活该功能，但母版页仅应用于顶级网站。有人能告诉我我的事件接收器有什么问题吗

   public override void FeatureActivated(SPFeatureReceiverProperties properties)
    {
        SPSite siteCollection = properties.Feature.Parent as SPSite;
        if (siteCollection != null)
        {
            SPWeb topLevelSite = siteCollection.RootWeb;

            // Calculate relative path to site from Web Application root.
            string WebAppRelativePath = topLevelSite.ServerRelativeUrl;
            if (!WebAppRelativePath.EndsWith("/"))
            {
                WebAppRelativePath += "/";
            }

            // Enumerate through each site and apply branding.
            foreach (SPWeb site in siteCollection.AllWebs)
            {
                site.MasterUrl = WebAppRelativePath + "_catalogs/masterpage/custom.master";
                site.CustomMasterUrl = WebAppRelativePath + "_catalogs/masterpage/custom.master";
                site.Update();
            }
        }
    }

任何帮助都会很好