Weired ssh-agent error in Ubuntu

To protect your private key, one might add a long pass-phrase for avoiding the burglary. However, somehow I see the following error while trying to add my private key to ssh-agent.

chucheng@ubuntuVM:~$ ssh-add
Could not open a connection to your authentication agent.

Here is the solution:
exec ssh-agent bash

Don't know why, but it just works :)

Results:

chucheng@ubuntuVM:~$ ssh-add
Enter passphrase for /home/chucheng/.ssh/id_rsa:
Identity added: /home/chucheng/.ssh/id_rsa (/home/chucheng/.ssh/id_rsa)