Swift Core Data - removeObject() in NSMutableOrderedSet unexpected behavior

Question

I have been chewing on this for a week now, and unless I am missing a very fundamental concept in Swift, this seems like a pretty serious bug.

I have a CoreData model with one-to-many relationships like so:

City -> has many Street

Also each entity has a name property of type string.

The corresponding classes are:

class City: NSManagedObject {

    @NSManaged var name: String
    @NSManaged var streets: NSOrderedSet
}

class Street: NSManagedObject {

    @NSManaged var name: String
}

And here comes the really weird stuff:

var city:City = getCities().first!
var streets = city.streets.array as! [Street]
var streetToDelete:Street? = nil

//This works
streetToDelete = streets.first

//This is not working !!!
for street in streets {
    if(street.name == "champs elysees"){
        streetToDelete = street
    }
}
deleteStreet(city, street:streetToDelete!) 

And the deleteStreet function:

func deleteStreet(var city:City, var street:Street){
    var streets = city.streets
    var streetsSet:NSMutableOrderedSet = streets.mutableCopy() as! NSMutableOrderedSet
    streetsSet.removeObject(street)
    city.streets = streetsSet.copy() as! NSOrderedSet
    saveContext()
} 

So if I use streets.first, the deletion works as accepted.
But if I use the foreach loop instead nothing happens - no error is thrown, and no street is deleted. (off course I need the foreach loop to find a specific street and delete it).

How come the .first and the foreach are different?
They suppose to return me a pointer to a street in both cases.

(getCities() is a simple, straight from the book, fetch operation)

Update: After further investigation, it seems that some how the iteration itself over the streets array causes the issue.