brew: brew uninstall -> Error: mpfr@2: undefined method `undent'

Please note we will close your issue without comment if you delete, do not read or do not fill out the issue checklist below and provide ALL the requested information. If you repeatedly fail to use the issue template, we will block you from ever submitting issues to Homebrew again.

• are reporting a bug others will be able to reproduce and not asking a question. If you’re not sure or want to ask a question do so on our Discourse: https://discourse.brew.sh
• ran a brew command and reproduced the problem with multiple formulae? If it’s a problem with a single, official formula (not cask) please file this issue at Homebrew/homebrew-core: https://github.com/Homebrew/homebrew-core/issues/new/choose. If it’s a brew cask problem please file this issue at https://github.com/Homebrew/homebrew-cask/issues/new/choose. If it’s a tap (e.g. Homebrew/homebrew-php) problem please file this issue at the tap.
• ran brew update and can still reproduce the problem?
• ran brew doctor, fixed all issues and can still reproduce the problem? (I’ve tried linking and unlinking mpfr@2 and mpfr but I till get the error)
• ran brew config and brew doctor and included their output with your issue?

To help us debug your issue please explain:

• What you were trying to do (and why) I’m trying to remove a package
• What happened (include command output) brew uninstall, reinstall and doctor exit with Error: mpfr@2: undefined method `undent' for #<String:0x0000000101161258>
• What you expected to happen The commands to succeed
• Step-by-step reproduction instructions (by running brew commands)

$ brew remove mpfr@2
Error: mpfr@2: undefined method `undent' for #<String:0x00000001025cc3f0>

$ brew config
HOMEBREW_VERSION: 1.7.0-13-g63c5bd9
ORIGIN: https://github.com/Homebrew/brew.git
HEAD: 63c5bd96a071074edc9c0e753e871faf1e8c8d1d
Last commit: 10 hours ago
Core tap ORIGIN: https://github.com/Homebrew/homebrew-core
Core tap HEAD: 6fb6e40e54fb48e14a4b98b32f805a2d3a4fd298
Core tap last commit: 74 minutes ago
HOMEBREW_PREFIX: /usr/local
HOMEBREW_DEV_CMD_RUN: 1
HOMEBREW_NO_ANALYTICS: 1
HOMEBREW_VISUAL: ne
CPU: octa-core 64-bit ivybridge
Homebrew Ruby: 2.3.7 => /usr/local/Homebrew/Library/Homebrew/vendor/portable-ruby/2.3.7/bin/ruby
Clang: 9.1 build 902
Git: 2.18.0 => /usr/local/bin/git
Curl: 7.54.0 => /usr/bin/curl
Java: 1.8.0_162
macOS: 10.13.6-x86_64
CLT: 9.4.1.0.1.1528165917
Xcode: 9.4.1
XQuartz: 2.7.11 => /opt/X11

$ brew doctor
Warning: You have unlinked kegs in your Cellar
Leaving kegs unlinked can lead to build-trouble and cause brews that depend on
those kegs to fail to run properly once built. Run `brew link` on these:
  mpfr@2
Error: mpfr@2: undefined method `undent' for #<String:0x0000000101dba338>