Swift Optionals issue when not returning nil

I ran across something that (I think) is weird. Is this expected behavior, or a bug or oversight in the Swift language?

When declaring a function that returns an optional, you're supposed to return nil in non-matching cases. This way, you can use the if let conditional to check for the optional.

But as in the example below, if instead of a nil I accidentally return a false value, no errors are thrown. And the if conditional always prints "is divisible". Maybe there is no way for the compiler to catch this, but it seems like a very tricky bug to have to track down (assuming a much more complex example). Or maybe it's just because this is such a simple example with a bool return?

Anyway, wanted to see if anyone thought this is just behaving as it should...

func isDivisible(#dividend: Int, #divisor: Int) -> Bool? {
    if dividend % divisor == 0 {
        return true
    }
    // An optional is supposed to return nil.
    return false
}

if let divisible = isDivisible(dividend: 42, divisor: 9) {
    println("is divisible")
} else {
    println("is not divisible")
}