Swift base class exceptions

OverridenFormatter.swift

/*
* Code to my question http://stackoverflow.com/questions/37448249/propagating-base-class-errors-in-swift
* I don't understand why there's such a difference in these two Formatters behavior
*/
import Cocoa

class OverridenFormatter: NSNumberFormatter {
    override func getObjectValue(obj: AutoreleasingUnsafeMutablePointer<AnyObject?>, forString string: String, range rangep: UnsafeMutablePointer<NSRange>) throws {
            // not doing anything, just delegating to the base class
            try super.getObjectValue(obj, forString: string, range: rangep)
    }
}

class NonOverridenFormatter: NSNumberFormatter {}

let nonOverridenformatter = NonOverridenFormatter()
let overridenFormatter = OverridenFormatter()

nonOverridenformatter.minimum = 15
overridenFormatter.minimum = 15

// WHY these calls yield different results ?
nonOverridenformatter.numberFromString("14") // return nil
overridenFormatter.numberFromString("14") // throws