Full Idea
Frege identifies each conceptual abstract with the corresponding extension of concepts. But the extensions themselves are among the abstracts, so each extension is identical with the class of all concepts that have that extension, which is absurd.
Gist of Idea
Frege gives an incoherent account of extensions resulting from abstraction
Source
comment on Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884]) by Kit Fine - The Limits of Abstraction I.2
Book Reference
Fine,Kit: 'The Limits of Abstraction' [OUP 2008], p.29
A Reaction
Fine says this point is 'from the standpoint of a general theory of abstracts', which presumably was implied in Frege, but not actually spelled out.