ERROR: no NOT NULL constraint found to drop

I run into error $subject with the query below:

# create table t1 (c int primary key null unique);
CREATE TABLE
# create table t2 (like t1);
CREATE TABLE
# alter table t2 alter c drop not null;
ERROR: no NOT NULL constraint found to drop

This starts since e056c557ae. I guess this shouldn't happen since the
comment says so.

/* this shouldn't happen */
elog(ERROR, "no NOT NULL constraint found to drop");

Thanks
Richard