changing default value in alter statement

[spatik]

Hi,
I have a table[event] with a column[event_description] as ntext and default value ''. Now, I want to change the column type to nvarchar(max) using this script,

alter table [event]
alter column [event_description] nvarchar(max) null ;

The event_description column was created with a default value ''.
Now this constraint is not allowing me to alter the column type. It throws an exception,

Msg 5074, Level 16, State 1, Line 1
The object 'DF__event__EVENT_DES__5A846E65' is dependent on column 'event_description'.
Msg 4922, Level 16, State 9, Line 1
ALTER TABLE ALTER COLUMN event_description failed because one or more objects access this column.

Is there any way to do this alteration?