Odd backtrace in condition-case

[Lars Ingebrigtsen]

I usually run with `debug-on-error' set, but lately I've started getting
these backtraces:

Debugger entered--Lisp error: (wrong-type-argument char-or-string-p nil)
  capitalize(nil)
  timezone-make-arpa-date(2000 0 0 "01:00:00" (3600 "CET"))
  timezone-make-date-arpa-standard("Session")
  date-to-time("Session")
  url-cookie-expired-p([url-cookie "NRK-Experiment-Bucket" "0" "Session" "/" 
"www.nrk.no" nil])

[...]

  url-http-create-request()
  url-http-async-sentinel(#<process www.nrk.no> "open\n")

The code is:

(defun url-cookie-expired-p (cookie)
  "Return non-nil if COOKIE is expired."
  (let ((exp (url-cookie-expires cookie)))
    (and (> (length exp) 0)
         (condition-case ()
             (time-less-p (date-to-time exp) nil)
           (error nil)))))

My question is: Why does this give me a backtrace?

If I say something else, like:

(condition-case ()
    (capitalize nil)
  (error t))

=> t

I get no backtrace, but:

(condition-case ()
    (date-to-time "Session")
  (error t))

=>
Debugger entered--Lisp error: (wrong-type-argument char-or-string-p nil)
  capitalize(nil)
  timezone-make-arpa-date(2000 0 0 "01:00:00" (3600 "CET"))
  timezone-make-date-arpa-standard("Session")
  date-to-time("Session")
  (condition-case nil (date-to-time "Session") (error nil))

*scratches head*

Shouldn't the `error' clause in condition-case catch all errors?

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no