want to get rid of an entry when using "add a tag"

Whenever i use this function, there is already an entry. I would like to get rid of that text. I have tried deleting it using the "-" button, it also disappears from the list, but the next time i use Alt-T to add a tag, it reappears. Seems like a bug to me. How can i clear this list? I would expect that field to be empty, when i use it.