This is surprising:

> db.collection.insert({a:1})
> db.collection.insert({})
> db.collection.distinct('a')
[ 1 ]
> db.collection.createIndex({a:1})
> db.collection.distinct('a')
[ null, 1 ]

A document without the 'a' field has no effect on the 'distinct' output when there's no index, but it contributes a 'null' to the output when there is an index.