"Hadrian's Forum" in Daily Kos Stumbles on "Who," "Whom"

From Daily Kos: Hadrian's Forum: Gracchus; Subverting Rome's Constitution, Part 1

To be a Tribune of the Plebs (also called the "People's Tribune"), one had to be a Pleb (although a couple of Tribunes in the later years of the Republic were Pactricii whom were adopted into a plebian family).

"Whom" in adjective subordinate clause should be "who," subject of "were adopted."