org-caldav: VTODO entries without DTSTART

[Tractatus <51183341@gmx.at>]

Hello,

I am testing syncing my ical files via Radicale CalDAV Server to org
usinf org-caldav. For some files I get an error "Format specifier
doesn't match argument type". [2]

In my ical files generated by Mozilla Icedove are VTODO entries [1]
without a DTSTART field. [1] I think this causes the error.
Since I do not care to sync VTODO'S is there a way to filter them out
before they are processed? Can this error be avoided?

The code seems to try to deal with missing dtstarts:
https://github.com/dengste/org-caldav/blob/master/org-caldav.el#L1198
but the error seems to occur before that:
https://github.com/dengste/org-caldav/blob/master/org-caldav.el#L1175


[1]
BEGIN:VTODO
CREATED:20150522T053330Z
LAST-MODIFIED:20150522T053352Z
DTSTAMP:20150522T053352Z
UID:b3a0257d-cf04-4675-825d-f47b802e5838
SUMMARY:test TODO
X-RADICALE-NAME:b3a0257d-cf04-4675-825d-f47b802e5838.ics
END:VTODO

[2]
Debugger entered--Lisp error: (error "Format specifier doesn't match
argument type") format("%02d:%02d" "nil" nil)
  icalendar--datetime-to-colontime(nil)
  (let* ((calendar-date-style ...) (ical-list ...) (e ...)
(zone-map ...) (dtstart ...) (dtstart-zone ...) (dtstart-dec ...)
(start-d ...) (start-t ...) (dtend ...) (dtend-zone ...)
(dtend-dec ...) (dtend-1-dec ...) end-d end-1-d end-t (summary ...)
(description ...) (rrule ...) (rdate ...) (duration ...)) (if (and
dtstart ...) (setq start-t nil)) (when duration (let ... ... ... ...))
(setq end-d (if dtend-dec ... start-d)) (setq end-1-d (if
dtend-1-dec ... start-d)) (setq end-t (if ... ... start-t)) (list
start-d start-t (if end-t end-d end-1-d) end-t summary description))
org-caldav-convert-event()