Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
ultrōnĕus, a, um, adj. [ultro], of one’s own accord, voluntary (late Lat.): exsilium, App. M. 1, p. 111, 15: ultroneam te dominae tuae reddis (i. e. ultro, tuā sponte), voluntarily, id. ib. 6, p. 175, 17; cf. id. Flor. 2, p. 350, 10; Vulg. Exod. 25, 2 (but in Sen. Q. N. 2, 59, 8, the correct read. is nati).