Can't get rid of constraint

Howdy,
I had a table in my database that reference another like:

create table a (1 int, 2 int);
create table b (1 int references a(1));

recently i had to do a:
drop table b;

now i can't:
delete from a where 1='5';

because i get:
ERROR: Relation 'b' does not exist

How do I get rid of this zombie constraint?

Thanks,
Matthew

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