diff --git a/Library/Homebrew/cask/migrator.rb b/Library/Homebrew/cask/migrator.rb
index 8df853aae8..c27c76aba6 100644
--- a/Library/Homebrew/cask/migrator.rb
+++ b/Library/Homebrew/cask/migrator.rb
@@ -80,7 +80,7 @@ module Cask
       when ".json"
         json = JSON.parse(path.read)
         json["token"] = new_token
-        json["old_tokens"] = (json["old_tokens"] << old_token).uniq
+        json["old_tokens"] = [old_token, *json["old_tokens"]].compact.uniq
         path.atomic_write json.to_json
       end
     end