Full Idea
Frege regarded the null set as an indefensible entity from the point of view of iterative set theory. It collects nothing.
Gist of Idea
The null set is indefensible, because it collects nothing
Source
report of Gottlob Frege (works [1890]) by Tyler Burge - Frege on Apriority (with ps) 2
Book Reference
Burge,Tyler: 'Truth, Thought, Reason (on Frege)' [OUP 2001], p.371
A Reaction
The null set defines the possibility that something could be collected. At the very least, it introduces curly brackets into the language.