Second Foundation

Plot hole: (Spoiler alert!) At the end of the first part ('Search by the Mule'), when the Mule, Channis and Pritcher are on the planet Rossem, the Mule says he had to follow Channis because Channis is an agent of the Second Foundation, and as such, has the ability to resist the Mule reading his mind. Yet, at the end, when the Mule's ships are bombed Tazenda, just before the First Speaker arrived, the mule forcibly made Channis tell where the Second Foundation is (that is, what Channis thought about the whereabouts of the Second Foundation). Now how come that Channis couldn't resist the Mule on Rossem, but he could on Kalgan? The Mule was much more powerful than Channis on either planets.