Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Īsēum, i, and Īsīum, ii, n., = Ἴσειον, the temple of Isis, Plin. 5, 9, 10, § 51; Plin. Ep. 10, 33, 1; Lampr. Alex. Sev. 26.