Oracle Message OCI-21522 - attempted to use an invalid connection in OCI (object mode only)

ERRORattempted to use an invalid connection in OCI (object mode only)
CAUSEUser attempted to use an invalid connection or a connection that has been terminated in an OCI environment (object mode), or user attempted to dereference a REF obtained from a connection which has been terminated.
ACTIONEnsure that the connection exists and is still valid.