Exception User-Unhandled, Creation of SPSite objects with a different site id or a different user token is not allowed

I am creating a event handler that will sync documents between two sites on the same site collection when one is uploaded. This is the item added handler.

    public override void ItemAdded(SPItemEventProperties properties)
    {
        using (DisabledItemEventsScope scope = new DisabledItemEventsScope())
        {
            try
            {
                using (SPWeb web = properties.Web)
                {
                    SPListItem doc = properties.ListItem;
                    String path = (String)doc[path_field];
                    SPFile file = (SPFile)web.GetFileOrFolderObject(path_field);

                    using (SPSite destSite = new SPSite(dest_url))
                    {


                        using (SPWeb destWeb = destSite.OpenWeb())
                        {
                            SPFolder destFolder = destWeb.Folders[dest_fold_url];

                            destWeb.AllowUnsafeUpdates = true;
                            destFolder.Files.Add(file.Name, file.OpenBinary(), true);
                            destFolder.Update();
                        }
                    }
                }
            }
            catch (Exception e)
            {
                properties.ListItem[error_field] = e.Message;
                properties.ListItem.Update();
            }

        }
    }

However I keep getting a Exception User-Unhandled : Exception has been thrown by the target of an invocation when I create the new site on the line using (SPSite destSite = new SPSite(dest_url)). I looked at the inner exception and it was this.

InnerException {"Specified argument was out of the range of valid values.\r\nParameter name: Creation of SPSite objects with a different site id or a different user token is not allowed."} System.Exception {System.ArgumentOutOfRangeException}

Any idea why this is happening and how to fix it?