Plenus can take an ablative (cf. Allen and Greenough, §409a) but it can also take an objective genitive (cf. §349a). I'm pretty sure the objective genitive is preferred (at least in poetry, according to A&G), but I could be wrong.

Allen and Greenough: https://books.google.com/books/about/Allen_and_Greenough_s_N..., see pages 216-217 (§349a) and 256 (§409a).