diff --git a/Library/Homebrew/utils/github.rb b/Library/Homebrew/utils/github.rb
index e893400da8..9b241f870b 100644
--- a/Library/Homebrew/utils/github.rb
+++ b/Library/Homebrew/utils/github.rb
@@ -47,7 +47,7 @@ module GitHub
   class AuthenticationFailedError < Error
     def initialize(github_message)
       @github_message = github_message
-      message += "GitHub #{github_message}:"
+      message = +"GitHub #{github_message}:"
       if ENV["HOMEBREW_GITHUB_API_TOKEN"]
         message << <<~EOS
           HOMEBREW_GITHUB_API_TOKEN may be invalid or expired; check: