Note in the appendix of "Grundlagen der Arithmetik" (Vol. 2), after Frege had received a letter of Bertrand Russell in which Russell had explained his discovery of, what is now known as, Russell's paradox, 1903.
Carl Gustav Hempel (2001). “The Philosophy of Carl G. Hempel: Studies in Science, Explanation, and Rationality”, p.24, Oxford University Press on Demand