Every now and then when provisioning a virtual machine with Vagrant I got an error during the provisioning process. Everything seemed to work fine but the error confused me and my developers so I was looking for the source of the error. I came a across a Vagrant bug report where Mitchell Hashimoto explained that "the reason this error message even appears is due to a bug in Ubuntu not checking whether stdin is a TTY, and just assuming it is". One of the comments in the discussion of this "bug" proposed a "bugfix" by adding the following line to the VagrantFile. Up to now this fix works fine for us and you might use it as well:
config.ssh.shell = "bash -c 'BASH_ENV=/etc/profile exec bash'"