Specified key was too long in mysql

[z...@ejustcn.com]

Hi,I have encountered an error when i run todoapp with mysql.Error message is : 
 Specified key was too long; max key length is 767 bytes.Then I trace into 
source and find auditentry class will create a  unique index with 
transactionid,target and propertyid,but field target type is varchar(2000) , it 
is too long to be an index.
so ,how can i ewsolve it?
Thanks !
James