Cannot Relate Look Table to Document Type

I am setting up a new R&D library. They would like to have attributes or
properties based off of the document type. Everytime I try to setup a
look table to use Document Type as the parents it clears it. If I
manually type Document Type in as the Related Table, it give the error:

"You may only relate a table to another table that exists, has no
parent, and is either Document Type or is user-defined table. Select a
table from the list."

Now it show Document type in the lookup selection but will not select
it. Is this a feature that was available at one time and removed or is
it a feature that has been disabled for some reason? Maybe a problem
with the snap-in.

Any Suggestions?