Seemingly valid cast gives confusing error message

What version of Racket are you using?

v7.0

What program did you run?

#lang typed/racket/base

(cast add1 (Number → Number))

What should have happened?

Cast succeeds.

If you got an error message, please include it here.

;  c:\Program Files\Racket\share\pkgs\typed-racket-lib\typed-racket\base-env\prims-contract.rkt:292:15: Type Checker: Error in macro expansion -- Type (case-> (-> Zero One) (-> One Positive-Byte) (-> Byte Positive-Index) (-> Index Positive-Fixnum) (-> Negative-Fixnum Nonpositive-Fixnum) (-> Nonpositive-Fixnum Fixnum) (-> Nonnegative-Integer Positive-Integer) (-> Negative-Integer Nonpositive-Integer) (-> Integer Integer) (-> Nonnegative-Exact-Rational Positive-Exact-Rational) (-> Exact-Rational Exact-Rational) (-> Nonnegative-Flonum Positive-Flonum) (-> Flonum Flonum) (-> Nonnegative-Single-Flonum Positive-Single-Flonum) (-> Single-Flonum Single-Flonum) (-> Nonnegative-Inexact-Real Positive-Inexact-Real) (-> Inexact-Real Inexact-Real) (-> Nonnegative-Real Positive-Real) (-> Real Real) (-> Float-Complex Float-Complex) (-> Single-Flonum-Complex Single-Flonum-Complex) (-> Inexact-Complex Inexact-Complex) (-> Number Number)) could not be converted to a contract: function type has two cases of arity 1
;   in: (U (case-> (-> Zero One) (-> One Positive-Byte) (-> Byte Positive-Index) (-> Index Positive-Fixnum) (-> Negative-Fixnum Nonpositive-Fixnum) (-> Nonpositive-Fixnum Fixnum) (-> Nonnegative-Integer Positive-Integer) (-> Negative-Integer Nonpositive-Integer...

Start with the failing cast example and inspect typed-racket/base-env/prims-contract.rkt around line 292, where the reported macro-expansion error originates. Determine why the shown function type cannot be converted to a contract, then verify that the example succeeds without the confusing error.