diff --git a/Library/Homebrew/cmd/test-bot.rb b/Library/Homebrew/cmd/test-bot.rb
index d4c9a6a615..cbe2a2698f 100644
--- a/Library/Homebrew/cmd/test-bot.rb
+++ b/Library/Homebrew/cmd/test-bot.rb
@@ -318,12 +318,13 @@ module Homebrew
       # Use Travis CI Git variables for master or branch jobs.
       elsif ENV["TRAVIS_COMMIT_RANGE"]
         diff_start_sha1, diff_end_sha1 = ENV["TRAVIS_COMMIT_RANGE"].split "..."
-        diff_start_sha1 = git("merge-base", diff_start_sha1, diff_end_sha1).strip
       # Otherwise just use the current SHA-1 (which may be overriden later)
       else
         diff_end_sha1 = diff_start_sha1 = current_sha1
       end
 
+      diff_start_sha1 = git("merge-base", diff_start_sha1, diff_end_sha1).strip
+
       # Handle no arguments being passed on the command-line e.g. `brew test-bot`.
       if no_args?
         if diff_start_sha1 == diff_end_sha1 || \