Failed to load uid from index in 0.19.6

Hi all,

Recently updated to 0.19.6 and we saw this error:

FetchPhaseExecutionException
Error
[[brew_staging-0][0]:
query[filtered(ConstantScore(NotDeleted(+cache(owner_id:[1 TO 1])
+cache(friend_score:[2.0 TO *])
+NotFilter(_uid:contact_document#1:1))))->cache(_type:contact_document)],from[0],size[300],sort[<custom:"score":
org.elasticsearch.index.field.data.floats.FloatFieldDataType$1@11e3853f>!,<custom:"created_at":
org.elasticsearch.index.field.data.longs.LongFieldDataType$1@7040f445>!]:
Fetch Failed [Failed to load uid from the index, missing internal _uid
field, current fields in the doc [[_source]]]]; }]

Rolled back to 19.4 and ran the same operation with no trouble. We're
not doing anything fancy here with the mappings. Has anyone else run
across this in 19.6?

Thanks,
Mat