UNION ALL query fails "while setting up Foreman"

The two sides of the UNION are not equivalent because one of the files has an additional column, but Aman tells me that this should work in a schema-less environment anyway. (I was expecting it to fail but with a more meaningful error because I am used to relational database UNION queries where a corresponding number of columns is expected on either side of the UNION operator.)

The query fails whether or not the LIMIT is applied. EXPLAIN for this query also fails.

0: jdbc:drill:> select * from `clicks/clicks.campaign.json` union all select * from `clicks/clicks.json` limit 5;
Query failed: Failure while setting up Foreman. DrillUnionRel#23471 [159a4852-e190-4edf-b11e-2d62db8efbd7]

Error: exception while executing query: Failure while trying to get next result batch. (state=,code=0)