User blog comment:Suparockr/List of Plotholes throughout the series/@comment-3228423-20120719212946/@comment-24174486-20120719223257

"They should've known that as a machine race, the geth are incapable of emotions and are therefore immune to charisma and lies"

"Charisma" doesn't just mean appealing to emotions. The Council believed that Saren was able to deceive the geth into believing about the Reapers like they believed that Saren had deceived Shepard. Being suspectible to lies is not entirely based on emotion—it is also based on whether or not you know better.