brew install and brew update hang with error: 'refs/remotes/origin/master': unknown revision or path not in the working tree

error: Not a valid ref: refs/remotes/origin/master fatal: ambiguous argument 'refs/remotes/origin/master': unknown revision or path not in the working tree.

 -----Solution------ 

rm -fr $(brew --repo homebrew/core) # because you can't `brew untap homebrew/core` 

brew tap homebrew/core