Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
interpŏlātĭo, ōnis, f. [interpolo], an alteration made here and there (post-Aug.): tenuatam (chartam) curiosa interpolatione, principalem fecit e plebeia, by a curious dressing made the best paper from common, Plin. 13, 12, 23, § 75; Tert. adv. Haer. 39.