I'd been having this issue whenever installing casks:

$ brew cask install macvim
==> This subcommand requires root privileges: bsexec
Error: Empty plist input

This was a known bug, and has been addressed with a newer version.

$ brew update
$ brew upgrade caskroom/cask/brew-cask

Be prepared for this to take a while.

==> Checking out tag v0.54.0
[..] 10M, built in 7.0 minutes

I've had mixed to positive success with Homebrew Cask, but it’s just useful enough to keep it around for installing Mac applications. XQuartz, Tunnelblick and a few others are still tricky, as they require admin access or wizard installs.