In this corrigendum, we offer a correction to [J. Korean Math. Soc. 54 (2017), No. 2, 461{477]. We construct a counterexample for the strengthened Cauchy{Schwarz inequality used in the original paper. In addition, we provide a new proof for Lemma 5 of the original paper, an estimate for the extremal eigenvalues of the standard unpreconditioned FETI-DP dual operator.