Getting Assertion when running Inner Merge Join query with SELECT COUNT(*) on top

When running the following query, it causing an ASSERTION (see attached log):

SELECT COUNT AS CNT FROM (SELECT ORDERS.O_ORDERKEY, ORDERS.O_ORDERDATE, ORDERS.O_CLERK, CUSTOMER.C_CUSTKEY, CUSTOMER.C_NAME, CUSTOMER.C_ACCTBAL
FROM TPCR01_PostgreSQLVirt.ORDERS, TPCR01_OracleVirt.CUSTOMER WHERE (CUSTOMER.C_CUSTKEY = ORDERS.O_CUSTKEY) AND
(ORDERS.O_ORDERDATE < '1992-01-05 00:00:00') AND (CUSTOMER.C_ACCTBAL < 9514.5)) AS T

Note: this doesn't fail when SELECT COUNT isn't used