Error when declaring polymorphic recursive let binding.

[YarinHeffes]

The following

(coalton-toplevel
  (define (f x)
    (let ((declare %f (Num :T => :T -> String))
          (%f (fn (x)
                (cond 
                  ((negative? x)
                   (f (+ 1 x)))
                  ((positive? x)
                   (f (- 1 x)))
                  (True
                   "Hooray!")))))
      (%f x))))

Results in the following error

Declared type ∀ :A. NUM :A ⇒ (:A → STRING) is more general than inferred type NUM :A ⇒ (:A → STRING).

However, the same code compiles when the type of %f is declared (UFix -> String), for example.