Inference for polymorphic functions not supported?

I keep getting the messages like

Type Checker: Inference for polymorphic keyword functions not supported in: (sort hash-list string<? #:key (λ ((p : (Pair Symbol Natural))) (symbol->string (car p))) #:cache-keys? #t)

or

Type Checker: Polymorphic function `cons' could not be applied to arguments:
Types: a (Listof a) -> (Listof a)
       a b -> (Pairof a b)
Arguments: (U Exact-Nonnegative-Integer Symbol) (Listof (Pairof Symbol Nonnegative-Integer))
Expected result: (Listof (Pairof Symbol Nonnegative-Integer))
 in: (for*/list : (Listof (Pair Symbol Natural)) ((p : (Pair Symbol Natural) (in-list (hash->sorted-list pname-map))) (k : Symbol (in-value (car p))) (v : Natural (in-value (cdr p))) #:when (not (= v 1)) (y (list k v))) y)

Is there some systematic way to code this explicitly so it 
doesn't have to try guesswork? (hich it refuses to do)

And is it really unable to figure out that an 
Exact-Nonnegative-Integer is a Nonnegative-Integer? 

-- hendrik

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