If you are installing using OSX homebrew you may need to set the $GOROOT in your .bashrc, .zshrc, etc:

export GOROOT=/usr/local/opt/go/libexec

I had the same error this morning when I updated from 1.9 -> 1.9.1 though according to several post the $GOROOT shouldn’t have to be set and I had not set it until today. This may be a bug?

Edit: not a bug, for more details see answer below.