MySQL de la conversion de décimal à chaîne

Pourquoi ne pas le CAST travail dans cette requête MySQL?

SELECT  MAX(Signups) AS Max, 
        MIN(Signups) AS Min,
        CAST(ROUND(AVG(Signups),2) AS VARCHAR(3)) AS Avg
FROM
(
    SELECT COUNT(1) AS Signups,
    DATE_FORMAT(JoinDate, "%Y-%m-%d") AS Date
    FROM users
    GROUP BY Date
) z 

Pourquoi j'obtiens cette erreur?

#1064 - You have an error in your SQL syntax; check the manual that corresponds 
to your MySQL server version for the right syntax to use near 
'VARCHAR(3)) AS Avg FROM ( ' at line 2