After successfully cloning my repo from heroku and added another remote
1/ git clone git@heroku.com:[APP].git 2/ git remote add bitbucket ssh://git@bitbucket
I had the same issue Kabir Sarin had. The solution was to clone the repo via SSH, instead of using the https URL. so this is what helped me, and hopefully others:
git clone git@bitbucket.org:{accountName}/{repoName}.git