diff --git a/Library/Homebrew/cmd/update.sh b/Library/Homebrew/cmd/update.sh
index 30cf94e13f..d0da09575c 100644
--- a/Library/Homebrew/cmd/update.sh
+++ b/Library/Homebrew/cmd/update.sh
@@ -258,7 +258,8 @@ EOS
 
   if ! git --version >/dev/null 2>&1
   then
-    brew install git
+    # we cannot install brewed git if homebrew/core is unavailable.
+    [[ -d "$HOMEBREW_LIBRARY/Taps/homebrew/homebrew-core" ]] && brew install git
     if ! git --version >/dev/null 2>&1
     then
       odie "Git must be installed and in your PATH!"
diff --git a/Library/Homebrew/utils/git.rb b/Library/Homebrew/utils/git.rb
index 21bb7695ce..25dc49b02c 100644
--- a/Library/Homebrew/utils/git.rb
+++ b/Library/Homebrew/utils/git.rb
@@ -7,6 +7,11 @@ module Utils
   def self.ensure_git_installed!
     return if git_available?
 
+    # we cannot install brewed git if homebrew/core is unavailable.
+    unless CoreTap.instance.installed?
+      raise "Git is unavailable"
+    end
+
     begin
       oh1 "Installing git"
       safe_system HOMEBREW_BREW_FILE, "install", "git"