Correlated Subquery with Join

I have 3 tables: alert, user_to_alert, and user_device. The device table holds a list of tokens and and includes a user_id field.

The user_to_alert table holds records of when a user reads an alert. If a record doesn't exist for the appropriate alert_id/user_id combo, it means the user hasn't read the alert. There is also an alert_read column that can be zero (if the user marks the alert as unread after reading it).

I have a query that loads a list of device tokens and I need to expand this to include the number of unread alerts. This is the query I've come up with

SELECT d.*, (
SELECT COUNT(*) FROM `alert` AS a 
LEFT JOIN `user_to_alert` AS uta ON 
( uta.`alert_id` = a.`alert_id` AND uta.`user_id` = d.`user_id` ) 
WHERE uta.`alert_read` = 0 OR uta.`alert_read` IS NULL) AS user_badge_number 
FROM `user_device` AS d  
WHERE d.`device_type` = 'iOS' AND d.`device_active` = 1 
GROUP BY d.`device_token`

This query gives me error #1054 - Unknown column 'd.user_id' in 'on clause'

I've setup an SQLFiddle with the schema and query: http://sqlfiddle.com/#!9/ee653/2

If I run my sub-query separately and substitute d.user_id for a valid user_id, I get the expected results (i.e. 3 for user_id 10). I assume this means the problem is where I'm referencing the column d.user_id. I guess MySQL is trying to perform the subquery first so d.user_id isn't a valid column yet? That's just a guess though.

I've tried moving the uta.user_id = d.user_id part of my ON clause to the WHERE clause but that just gives all rows a 0 for the user_badge_number column.

I'm having trouble finding anything via Google that relates to this issue. Everything I find about correlated subqueries seems to be using them in ON or WHERE clauses instead of using them to retrieve extra data.

This is being run via a PHP script, so I know that I could run the sub query by itself within the result loop of the PHP script. Obviously, I'd rather figure out how to do it with one query as that would be more efficient.