latest snapshot crashes backend

The attached commands crash the backend, which exits with status 11. The
backend log (debug level 255) does not show anything that is meaningful to
me. It shows a query, which seems to relate to the final constraint, and
then terminates without further explanation:

...
query: select 1 from individual where NOT ( surname IS NULL AND forenames IS
NUL
L )
parser outputs:
{ QUERY
...
}
after rewriting:
{ QUERY
...
}
/usr/lib/postgresql/bin/postmaster: reaping dead processes...
/usr/lib/postgresql/bin/postmaster: CleanupProc: pid 8970 exited with status 11
...

I don't know where to start looking. I can trace the backend with the
debugger if you will tell me which routine to break at.