Minor buglet in update...from (I think)

A minor nasty error I got when trying to improve the query used to disable
triggers:

create table t1(f1 int4, f2 int4);
create table t2(f1 int4, f2 int4);

insert into t1 values(1, 0);
insert into t1 values(2, 0);

insert into t2 values(1, 0);

update t1 set f2=count(*) from t2 where t1.f1=1 and t2.f1=t1.f1 ;
UPDATE 1

update t1 set f2=count(*) from t2 where t1.f1=2 and t2.f1=t1.f1 ;
ERROR: ExecutePlan: (junk) `ctid' is NULL!

I would have expected no update to occur since no rows match.

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