Change column type to numeric

Forgive me if this question has an obvious answer, I'm sorta new to posgresql.

I have a table that's already populated with quite a bit of records.
I'd like to alter a column called "amount" from character varying to
numeric, so I don't have to re-load all of my data sets.

I've already dropped the column default. When attempting to change
the column type, I get:

test=# alter table foo alter column amount type numeric(10,2) USING
cast(amount AS numeric);
ERROR: invalid input syntax for type numeric: ""

I'm assuming that it's trying to cast a blank value as numeric and
failing. Does anyone know of an easy way to work around this?

Thanks!

--Jake