Odd termination

In the following join:
select * from computer,ethernet where computer.ethernet=ethernet.id;
One of the instances of computer has no ethernet value. I thought
computer.ethernet=ethernet.id would just evaluate to false for this
instance, yet

pqReadData() -- backend closed the channel unexpectedly.
This probably means the backend terminated abnormally before
or while processing the request.
We have lost the connection to the backend, so further processing is
impossible. Terminating.

happened instead. Is this right?

Cheers,

Patrick

(source supped Sep 9)