Full Idea
Since he thought of classes as extensions of concepts, ...it is because a concept can be empty that there is such a thing as the empty class.
Gist of Idea
It is because a concept can be empty that there is such a thing as the empty class
Source
report of Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by Michael Dummett - Frege philosophy of mathematics Ch.8
Book Reference
Dummett,Michael: 'Frege: philosophy of mathematics' [Duckworth 1991], p.92
A Reaction
Frege was already up against the awaiting Russell Paradox, but this view also seems to imply that there are many empty classes, since the absences of sandwiches would be different from the absence of heroism.