Full Idea
The null class is useful in formal set theory, but I hope that does not require that there be a thing called the null class which is truthmaker for the strange proposition
Gist of Idea
For 'there is a class with no members' we don't need the null set as truthmaker
Source
David M. Armstrong (Truth and Truthmakers [2004], 09.1)
Book Reference
Armstrong,D.M.: 'Truth and Truthmakers' [CUP 2004], p.114
A Reaction
It is not quite clear why it doesn't, but then it is not quite clear to philosophers what the status of the null set is, in comparison with sets that have members.