You'll notice there's also a problem with it not realizing that the unless terms will stop execution before I try to return non-string values.

gistfile1.rkt

#lang typed/racket

(require racket/cmdline)

;; The command line syntax is currently:
;;   spims  -p <file> -s <file>
;; where the first file is a pattern image and the second file is a source image

(define-type MaybeString [U False String])

(: parse-arguments ([Vector String] -> [Values String String]))
(define (parse-arguments arguments)
 (let: ((pattern-image-filename : MaybeString #f)
        (source-image-filename : MaybeString #f))
   ;; This expression returns (void) because there is no #:args clause. A #:args
   ;; clause could be used to parse arguments which succeed the regular, flagged
   ;; arguments.
   (command-line #:program "spims"
                 #:argv arguments
                 #:once-each
                 ["-p" filename "the pattern image"
                  (set! pattern-image-filename filename)]
                 ["-s" filename "the source image"
                  (set! source-image-filename filename)])
   ;; verify that the necessary arguments were passed
   (unless source-image-filename
     (error 'parse-arguments
            "You must specify a source image -- see 'spims -h' for help."))
   (unless pattern-image-filename
     (error 'parse-arguments
            "You must specify a pattern image -- see 'spims -h' for help."))
   ;; this returns two values, you can use a (let-values (((a b) ...) ...) ...)
   ;; form to capture these values
   (values pattern-image-filename source-image-filename)))

gistfile2.txt

I get these errors:

racket parse-command-line-typed.rkt
parse-command-line-typed.rkt:22:18: Type Checker: Mutation only allowed with compatible types:
Any is not a subtype of MaybeString
  in: (set! pattern-image-filename filename)
  context...:
   /usr/local/lib/racket/collects/typed-racket/utils/tc-utils.rkt:95:12: for-loop
   f16
   /usr/local/lib/racket/collects/typed-racket/typecheck/tc-toplevel.rkt:295:0: type-check
   success
   /usr/local/lib/racket/collects/typed-racket/typed-racket.rkt:40:4
   standard-module-name-resolver
parse-command-line-typed.rkt:24:18: Type Checker: Mutation only allowed with compatible types:
Any is not a subtype of MaybeString
  in: (set! source-image-filename filename)
  context...:
   /usr/local/lib/racket/collects/typed-racket/utils/tc-utils.rkt:95:12: for-loop
   f16
   /usr/local/lib/racket/collects/typed-racket/typecheck/tc-toplevel.rkt:295:0: type-check
   success
   /usr/local/lib/racket/collects/typed-racket/typed-racket.rkt:40:4
   standard-module-name-resolver
parse-command-line-typed.rkt:34:11: Type Checker: Expected String, but got MaybeString
  in: pattern-image-filename
  context...:
   /usr/local/lib/racket/collects/typed-racket/utils/tc-utils.rkt:95:12: for-loop
   f16
   /usr/local/lib/racket/collects/typed-racket/typecheck/tc-toplevel.rkt:295:0: type-check
   success
   /usr/local/lib/racket/collects/typed-racket/typed-racket.rkt:40:4
   standard-module-name-resolver
parse-command-line-typed.rkt:34:34: Type Checker: Expected String, but got MaybeString
  in: source-image-filename
  context...:
   /usr/local/lib/racket/collects/typed-racket/utils/tc-utils.rkt:95:12: for-loop
   f16
   /usr/local/lib/racket/collects/typed-racket/typecheck/tc-toplevel.rkt:295:0: type-check
   success
   /usr/local/lib/racket/collects/typed-racket/typed-racket.rkt:40:4
   standard-module-name-resolver
Type Checker: Summary: 4 errors encountered
  context...:
   /usr/local/lib/racket/collects/typed-racket/typecheck/tc-toplevel.rkt:295:0: type-check
   success
   /usr/local/lib/racket/collects/typed-racket/typed-racket.rkt:40:4
   standard-module-name-resolver

Compilation exited abnormally with code 1 at Sat Feb  9 17:54:31