brew update で to_sym エラーが出た時の対処方法

エラー内容

以下がでた

% brew update
Error: undefined method `to_sym' for nil:NilClass
Please report this bug:
    https://github.com/mxcl/homebrew/wiki/troubleshooting
/usr/local/Library/Homebrew/cmd/update.rb:134:in `report'
/usr/local/Library/Homebrew/cmd/update.rb:132:in `each_line'
/usr/local/Library/Homebrew/cmd/update.rb:132:in `report'
/usr/local/Library/Homebrew/cmd/update.rb:36:in `update'
/usr/local/Library/brew.rb:91:in `send'
/usr/local/Library/brew.rb:91

対処方法

% brew doctor
Your system is ready to brew.
$ brew update
Updated Homebrew from XXXXXXXX to XXXXXXXX.
==> Updated Formulae