tooltips missing sometimes

In DrRacket, this program:

#lang typed/racket #:no-optimize

(: plus (-> (U String Integer)
            (U String Integer)
            (U String Integer)))
(define (plus x y)
  (cond
    [(and (number? x) (number? y))
     (+ x y)]
    [(number? x)
     (string-append
      (format "~a" x)
      (format "~a" y))]
    [(string? x)
     (string-append
      (format "~a" x)
      (format "~a" y))]
    [else
     (+ x (car x))]))

shows no tooltips in the (+ x (car x)) bit. It would be nice to show something about what's going on (not just silence).