Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† Hercēus, i, m., = Ἑρκεῖος (of or belonging to the court-yard), an epithet of Jupiter, as the protector of the house and its enclosure (pure Lat. Penetralis): Herceus Juppiter intra conseptum domus cujusque colebatur, quem etiam deum penetralem appellabant, Paul. ex Fest. p. 101 Müll. N. cr.; Hyg. F. 91; Sen. Agam. 469; cf. Serv. Verg. A. 2, 469: ara Hercei Jovis, Ov. Ib. 286 (al. Rhoetei).
Hence, also: Herceae arae, Luc. 9, 979 (dub.).