Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
castŏrĕum, ei, n. [1. castor], castor or castoreum, a secretion of the beaver having a strong odor, Plin. 8, 30, 47, § 109; Lucr. 6, 794; Cels. 6, 7, 8; Scrib. 3.
In plur., Verg. G. 1, 59; Plin. 32, 9, 31, § 101; Nemes. C. 224.