Replace Blank Album with Text

I'm trying to replace blank albums with an album name by defining an action that replaces a blank with a name, but mp3tag won't let me save the action with a null Original name. How can I accomplish what I want?

"What do you mean by that " ?

This should you get started
Create an Action called "No Album then Unknown Album"

Format Value: ALBUM
Format String: $IF(%ALBUM%,%ALBUM%,Unknown Album)

What it does is:
If the %ALBUM% Tag is empty it will write "Unknown Album" in the ALBUM Tag.

The window where the action is defined doesn't allow me to leave the Original field blank. If a value isn't provided, clicking OK doesn't close the window.

Exactly what I needed. Thanks!