Cascading deletes with rules in 7.0.3: why doesn't this work?

In the following example, I'm trying to log all deletions from any of three
tables. The only information I'm interested in recording is the index from
table A which the deleted row referenced.

When I try to delete a row from A or B:

testdb=> delete from b;
ERROR: ExecAppend: Fail to add null value in not null attribute ax
testdb=> delete from c;
DELETE 1
testdb=> delete from b;
DELETE 1

What's really odd is that if I drop the *_delete rules, and try 'delete
from a' in the same psql session, I get that ExecAppend error. But
'delete from a' works in a new session:

testdb#1=> delete from a;
ERROR: ExecAppend: Fail to add null value in not null attribute ax
testdb#1=> drop rule a_delete;
DROP
testdb#1=> drop rule b_delete;
DROP
testdb#1=> drop rule c_delete;
DROP
testdb#1=>
[1]+ Stopped psql -d testdb -U tester
postgres# psql -d testdb -U tester
testdb#2=> delete from a;
DELETE 1
testdb#2=> \q
postgres# fg
testdb#1=> insert into a values (1);
INSERT 1961282 1
testdb#1=> insert into b values (1,1);
INSERT 1961283 1
testdb#1=> insert into c values (1,1);
INSERT 1961284 1
testdb#1=> delete from a;
ERROR: ExecAppend: Fail to add null value in not null attribute ax
testdb#1=>

Does anyone know what the problem is?

Thanks,

Jeremy

-- cut here --

create table a (
ax serial,
primary key (ax)
);

create table b (
bx serial,
ax int references a on delete cascade,
primary key (bx)
);

create table c (
cx serial,
bx int references b on delete cascade,
primary key (cx)
);

create table delete_log (
ax int not null default '0',
delete_timestamp timestamp default current_timestamp
);

create rule a_delete as
on delete to a do
insert into delete_log (ax) values (old.ax);

create rule b_delete as
on delete to b do
insert into delete_log (ax) values (old.ax);

create rule c_delete as
on delete to c do
insert into delete_log (ax)
values ((select ax from b where b.bx = old.bx));

insert into a values (1);
insert into b values (1,1);
insert into c values (1,1);