The ability to remove the series number

A feature that I would love to see in calibre would be to have the option to remove the series number completely on some books in the library.
I have a large amount of academic history books in my library, and frequently University Press books have a series, but no series number (Palgrave Macamillan's The New Middle Ages, for example.) I currently set the series number to 0.00, but would love to have the option to have it show up as undefined, like can be done with the date published field (using the little recycle button.)

This is a "it would be nice to have" type of request, not a "I gotta have it, or my world is gonna end" request, so no huge rush.