Could not open a connection to your authentication agent

Question

I am getting this warning error:

$ git push heroku master
Warning: Permanently added the RSA host key for IP address '50.19.85.132' to the list of known hosts.
!  Your key with fingerprint b7:fd:15:25:02:8e:5f:06:4f:1c:af:f3:f0:c3:c2:65 is not authorized to access bitstarter.

Then when I add the keys I get this error: 

$ ssh-add ~/.ssh/id_rsa.pub
Could not open a connection to your authentication agent.

Does anyone have any idea about this?