Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
rastellus, i, m. dim. [rastrum], a hoe, rake, mattock, Varr. L. L. 5, 31, 38; id. R. R. 1, 49, 1; Col. 2, 12, 6 (for which, id. 2, 10, 27, rastra), Suet. Ner. 19.