Playing around with Nat literals and I'm finding cases (as below) where everything goes kind of sideways. Any clues?

gistfile1.scala

// Scala 2.10.2 and Shapeless 2.0.0-SNAPSHOT

import shapeless._
import shapeless.nat._

sealed trait Month {
  type ND <: Nat
  def apply(n: Nat)(implicit ev: LTEq[n.N, ND]) = "foo"
}

def mkMonth(len: Nat) = 
  new Month {
    type ND = len.N
  }

val Jan = mkMonth(31)

Jan(30) // compiles ok
Jan(31) // spins the compiler; never returns