Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
dissertātĭo, ōnis, f. [disserto], a spoken dissertation, discourse, disquisition (not a written treatise; cf. disputatio; late Lat.), Gell. 1, 2, 6; 10, 4, 1; 14, 3, 5 (in Plin. 10, 68, 87, § 190, the true reading is edissertatio) al.