Alter table adding default value

Hi Guys

I try to add a default value to a table using this script:

ALTER TABLE "a" ALTER ("b" varchar DEFAULT '');

My goal is to set empty string default value for the field b

When I try to run this script:

feature not supported: cannot shorten the field length: b:

My b field is :

....

"b" VARCHAR(12) NOT NULL

...

Any hints?

Thanks in advance