COALESCE woes

Hi PG List,

I'm missing something or haven't had enough coffee yet. What gives with
the COALESCE in the view below?

mxl_sqr=# \d users
Table "public.users"
Column | Type | Modifiers
---------+---------+-----------
user_id | integer | not null
Indexes:
"users_pkey" PRIMARY KEY, btree (user_id)

CREATE TABLE ts1 (
user_id int references users(user_id),
ts timestamptz default now()
);

CREATE TABLE ts2 (
user_id int references users(user_id),
ts timestamptz default now()
);

CREATE TABLE ts3 (
user_id int references users(user_id),
ts timestamptz default now()
);

CREATE OR REPLACE VIEW user_timestamps
AS
SELECT COALESCE(ts1.user_id, ts2,user_id, ts3.user_id) AS user_id,
max(ts1.ts) AS ts_x,
max(ts2.ts) AS ts_y,
max(ts3.ts) AS ts_z
FROM ts1
LEFT JOIN ts2 USING (user_id)
LEFT JOIN ts3 USING (user_id)
GROUP BY 1;
ERROR: COALESCE types integer and ts2 cannot be matched
*LINE 3: SELECT COALESCE(ts1.user_id, ts2,user_id, ts3.user_id) AS us...*

* ^*

All types match from start to finish.

Thanks,
-Greg