“Credulousness” for “credulity.”
Eugene Volokh seems to be quoting Clayton Cramer, but I can’t find the monster in the Cramer post.
Update Eugene reports that he was quoting the original title — since changed — of the Cramer post. He also points out that “credulousness” is attested as far back as 1598, only half a century after the first recorded appearance of “credulity.” Isn’t it amazing that they were writing English that barbarously while Shakespreare was still alive?