Did you get Monty from a petstore or breeder?
If it was from a breeder, I'd be willing to bet it's still a male and just didn't show you his hemipenes. Once a breeder pops and see's them, it's a guarenteed male, but sometimes mistakes happen with females because the hemipenes don't show and they think it's a female when it's actually a male.

So you see, males are rarely mistaken if that's what they're labeled as.