[racket] Getting an expression to type

I'm having difficulty getting the following code to type:

#lang typed/racket
(: test : ((U Number (Parameterof Number)) -> Number))
(define (test p)
  (if (not (parameter? p))
      p
      (p)))

But I get the error:
> Type Checker: Expected Number, but got (U Complex (Parameterof Number))
in: p
On the then branch of the if. Is there a way to convince this to type?

--Spencer
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