read-all receiving #<unspecified> unless preceded by a precidate

[sjamaan]

This happens in csi:

;; This works:
(require-extension utils)

(define (command/string command)
  (define-values (proc-output proc-input proc-id) (process command))
  (port? proc-output)
  (define retval (read-all proc-output))
  (close-input-port proc-output)
  (close-output-port proc-input)
  retval)

(print (command/string "ls"))

;; This breaks:

(require-extension utils)

(define (command/string command)
  (define-values (proc-output proc-input proc-id) (process command))
  (define retval (read-all proc-output))
  (close-input-port proc-output)
  (close-output-port proc-input)
  retval)

(print (command/string "ls"))

;; Gives us:
;; Error: (open-input-file) bad argument type - not a string: #<unspecified>