Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
infĭdēlĭtas, ātis, f. [infidelis], unfaithfulness, faithlessness, infidelity: amicitiarum, Cic. Tusc. 5, 22: infidelitatis suspicionem sustinere, * Caes. B. C. 2, 33.
In plur.: quantae infidelitates in amicitiis, Cic. Mil. 26, 69.