validates_uniqueness_of and MySQL enum type

I tried to check validates_uniqueness_of for a MySql enum type column
with a scope given, like this:

validates_uniqueness_of :phone_type, :scope => :user_id

And It didn't work. When I check the SQL statement for doing this
validation, it is like this:

SELECT `phone_numbers`.id FROM `phone_numbers` WHERE
(`phone_numbers`.`phone_type` = BINARY '' AND `phone_numbers`.user_id
= 23) LIMIT 1

phone_type was set to be "Home". If I try validates_uniqueness_of on
any other columns. it works fine. Could some please highlight me
what's going on? Thanks