Did Gandalf make a mistake at Isengard ?

[Hakon]

Originally Posted by The Mouth of Sauron

A Nazgul could have snatched them easily enough, as they sat alone by the gate, while the Ents were working at the north side of Isengard.

I think even the Nazgul fear the Ents, they would not venture towards Isengard if they were aware of the Ents there.