Creating list item of type Discussion programmatically

I am trying to create list item in SP list Discussion. New item is created, but, my Root Folder in that list is different from item title, so that discussion is no good, when I try to add new answer in that Discussion I get error: "Value does not fall in the expected range".

Root Folder has name like 1_.000

How can I create Root Folder of that item? Or am I doing something else wrong? My code:

SPList discussionList = oWeb.Lists["Discussion"];

SPListItem listItem = tDiscussionList.Items.Add();
listItem[SPBuiltInFieldId.Title] = "some title";
oWeb.AllowUnsafeUpdates = true;
listItem.Update();