ON CONFLICT ON CONSTRAINT fails saying constraint doesn't exist

I'm trying to use new Postgresql 9.5 upsert feature. But for some reason my query saying constraint doesn't exist (when it does).

My query is this

INSERT INTO journals (ext_ids, title) VALUES ('{"nlmid": "000"}', 'blah')
ON CONFLICT ON CONSTRAINT idx_nlmid_journal DO NOTHING;

where idx_nlmid_journal is unique index on jsonb field created like this

CREATE UNIQUE INDEX idx_nlmid_journal ON public.journals ((ext_ids ->> 'nlmid'::text));

I get error

ERROR: constraint "idx_nlmid_journal" for table "journals" does not exist

what am I missing ?

The syntax you use is not valid for a unique index because a unique index does not create a constraint. You need to remove the ON CONSTRAINT and use the index expression instead.

This works:

INSERT INTO journals (ext_ids, title) 
VALUES ('{"nlmid": "000"}', 'blah')
ON CONFLICT ((ext_ids ->> 'nlmid'::text)) 
DO NOTHING;

StackExchange Database Administrators Q#139756, answer score: 14