GetOldestXmin() is not allowed on a hot standby

[anse1]

smith@moll:~$ psql -U smith -p 54331 -h /tmp regression
psql (9.6.3, server 10beta2)
regression=> select * from pg_dirtyread('tenk1'::regclass) t(dead boolean, ctid tid);
server closed the connection unexpectedly
	This probably means the server terminated abnormally
	before or while processing the request.
smith@moll:~$ grep TRAP log-slave
TRAP: FailedAssertion("!(allDbs || !RecoveryInProgress())", File: "procarray.c", Line: 1332)