how to do prefix_range default value?

hi,all
     I have installed prefix contrib,it works well.
     now I want to insert default value [] to prefix column,because if the prefix column is NULL,the returns are not I want (with prefix @>'text').I alter column prefix default [],it return error:
ERROR:  syntax error at or near "["
LINE 2:    ALTER COLUMN prefix SET DEFAULT [];

plz,how can I alter default value []?

thanks