Getting "invalid cache ID: 42" in pg10.5

PostgreSQL version: 10.5 Operating system: CentOS release 6.9 (Final)

Hi We're using 10.5 ,i get the error: report=> select count(1) from monitor_trade_detail where mernum_maps is not null;ERROR: invalid cache ID: 42CONTEXT: parallel worker
the table data count 606333
report=> \dx List of installed extensions Name | Version | Schema | Description ------------+---------+------------+----------------------------------------------------------------------------------- jsquery | 1.1 | report | data type for jsonb inspection oracle_fdw | 1.1 | report | foreign data wrapper for Oracle access plpgsql | 1.0 | pg_catalog | PL/pgSQL procedural language tds_fdw | 1.0.8 | report | Foreign data wrapper for querying a TDS database (Sybase or Microsoft SQL Server)(4 rows)
It's only happening occasionally (on a not very busy system).
Many thanks