What could cause CREATE TEMP... "could not read block" error?

Howdy. I have two tables that I'm joining together and using the result to
create a temporary table. Performing the join operation works fine; it
produces 0 rows (as expected). If I prepend "CREATE TEMP tmp_policyqueue
AS" to the SELECT then it generates this error:

ERROR: could not read block 39 in file "base/16391/11937": read only 0 of
8192 bytes

$ psql
psql (9.3.9)
mdb=> CREATE TEMP TABLE tmp_policyqueue AS
mdb-> SELECT pq.* FROM policyqueue AS pq
mdb-> JOIN seed_progress AS sp ON pq.id=sp.polidx;
ERROR: could not read block 40 in file "base/16391/11937": read only 0 of
8192 bytes

You'll also observe that the block number is changing each time I execute
the command. I know very little about postgres internal structure so it may
be irrelevant. I've left my database in this state should extra information
be needed.

Thanks for any assist.

Chris