Problem UPSERTing with partial unique indexes

[S4G4R]

Given a table definition such as :

create table sample_relation
(
  id    text not null,
  b_id  int  not null,
  c_id  int,

  unique(id, b_id, c_id),
  foreign key (id)   references sample_relation_2(id),
  foreign key (b_id) references sample_relation_3(id),
  foreign key (c_id) references sample_relation_3(id)
);

and a partial unique index defined on it :

create unique index idx_sample_relation_id_b_id
           on sample_relation(id, b_id)
        where c_id is null;

I would like to UPSERT some data. I have tried doing that in the following way :

(sql/sql
 (sql/insert db :sample-relation []
             (sql/values {:id "abc"
                          :b-id 1
                          :c-id 2})
             (sql/on-conflict [:id :b-id]
                              (sql/where '(is-null c-id))
                              (sql/do-update {:c-id :EXCLUDED.c-id}))))

which should translate to this query :

insert into sample_relation(id, b_id, c_id)
     values ('abc', 1, 2)
on conflict (id, b_id)
      where c_id is null
  do update set c_id = EXCLUDED.c_id;

but instead wrongly translates to (Note the position of the WHERE clause) :

insert into sample_relation(id, b_id, c_id)
     values ('abc', 1, 2)
on conflict (id, b_id)
  do update set c_id = EXCLUDED.c_id
      where c_id is null;

which in turn gives me the error :

column reference "c_id" is ambiguous

Is my syntax wrong or is this a bug?