original brew update Error needs to run git fetch --unshallow, but got another git fatal

$ brew update
Error: homebrew-core is a shallow clone. To `brew update` first run:
  git -C "/usr/local/Homebrew/Library/Taps/homebrew/homebrew-core" fetch --unshallow

I tried to run above command: git ... --unshallow but I got another error:

fatal: dumb http transport does not support shallow capabilities

$ brew -v
Homebrew 2.6.0-104-g24f7898
Homebrew/homebrew-core (git revision b1ef15; last commit 2020-12-05)
Homebrew/homebrew-cask (git revision 443e7; last commit 2020-08-27)

Anyone can help?

I dug a little after trying to deal with the git repo, but the simplest thing that fixed it for me was untapping and then retapping the casks repo

brew untap homebrew/cask
brew tap homebrew/cask

that put everything back in order.

I haven't RTFM'd lately, but I'm guessing this has something to do with how casks seem to be moving into the main homebrew repo (or at least that's how it seems as a naive user).

I tried to run: $ git -C "/usr/local/Homebrew/Library/Taps/homebrew/homebrew-core" fetch

then: $ git -C "/usr/local/Homebrew/Library/Taps/homebrew/homebrew-core" fetch --unshallow

brew update seems OK now.