Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
crustum, i, n. [access. form to crusta], pastry, any thing baked, Hor. Ep. 1, 1, 78; Verg. A. 7, 115; Inscr. Grut. 173, 4; 373, 8.