diff --git a/Library/Homebrew/cmd/update-report.rb b/Library/Homebrew/cmd/update-report.rb
index 01ca61f9bd..ee5be24c5c 100644
--- a/Library/Homebrew/cmd/update-report.rb
+++ b/Library/Homebrew/cmd/update-report.rb
@@ -575,7 +575,7 @@ class Reporter
     @diff ||= if installed_from_api?
       # Hack `git diff` output with regexes to look like `git diff-tree` output.
       # Yes, I know this is a bit filthy but it saves duplicating the #report logic.
-      diff_output = Utils.popen_read("git", "diff", api_names_before_txt, api_names_txt)
+      diff_output = Utils.popen_read("git", "diff", "--no-ext-diff", api_names_before_txt, api_names_txt)
       header_regex = /^(---|\+\+\+) /.freeze
       add_delete_characters = ["+", "-"].freeze