[Avodah] Ramban on Taking a Census

[Ben Waxman via Avodah]

The Ramban says that David HaMelech made a mistake and thought that the 
issur of taking a direct census was a mitzvah for the Dor HaMidbar only.

How does David HaMelech make a mistake like that???? And if he made a 
mistake, how is it that no one corrected him? Or was David like other 
kings who simply were unwilling to listen to anyone, including a navi?

Ben