DISTINCT does not return compressed columns

I have a simple query listed below which correctly returns 20 rows of data. If I add DISTINCT to the clause, it still returns 20 rows but the "purchase_token" field is empty for all rows. This field, "purchase_token" is a compressed field, the only one returned in this query.

Working query:
select uid, order_id, purchase_token from ud_registration where is_consumable = 1;

Failing query:
select DISTINCT uid, order_id, purchase_token from ud_registration where is_consumable = 1;