INSERT with on duplicate key update

INSERT with on duplicate key update

I'm having difficulty understanding the correct syntax for an insert with
an on duplicate key update check.
I currently get an error with the query:
INSERT INTO users_items (q,id,uid)
ON DUPLICATE KEY UPDATE
q = q + ?, id = ?, uid = ?
The unique key is uid + id together.
My error is:
Syntax error or access violation: 1064 You have an error in your SQL syntax;