Are there null arguments in Athabascan?

The existence of null arguments in Athabascan languages, and other polysynthetic languages, is a debated topic. We argue that, if the agreement on the verb is the argument, the so-called Pronominal Argument Hypothesis, languages such as Navajo do not have null arguments in the traditional sense. We then outline the diachrony of Athabascan object marking on the verb (by looking at the various contemporary languages).