This is an archived snapshot of W3C's public bugzilla bug tracker, decommissioned in April 2019. Please see the home page for more details.
Argument is defined as: typedef long or boolean or DOMString or WebElement or Object or sequence Argument; So Argument doesn't allow null/dictionary/dictionary. But later in the section it's said: 1. a long, boolean, DOMString, or null ... 2. an array 3. a dictionary Note that typedef allows Argument to be of type sequence but such ability doesn't seem to be used.
landed in https://dvcs.w3.org/hg/webdriver/rev/1bfbe3b03220