negative values for EFFORT result in error when switching to column view

[Heiko Schmidt <Heiko.Schmidt@webbedtables.de>]

Hi everybody.

When planning projects I estimate the amount of time for tasks by
filling the property EFFORT with an amount of time.


#+begin_src org
     ,*** Projekt
             :PROPERTIES:
             :COLUMNS:  %30ITEM %8EFFORT{:}
             :END:
     ,**** effort planned/estimated
              :PROPERTIES:
              :EFFORT:   10 h
              :END:
#+end_src

After acceptance of this planned time the project will be executed and
I sum the amount of time I really needed.

#+begin_src org
     ,**** first effort done
              :PROPERTIES:
              :EFFORT:   -2 h
              :END:
     ,**** secon effort done
              :PROPERTIES:
              :EFFORT:   -3 h
              :END:
#+end_src

When M-x org-columns I'd like to see column view with the balanced sum
of the hours planned and the hours really worked, but I get an error

cond: Invalid duration format: "-2 h"

I know I'm using old versions but this problem is also on newer versions.

org-version: 9.2
GNU Emacs 24.4.1 (x86_64-pc-linux-gnu, GTK+ Version 3.14.5)
  of 2017-09-12 on hullmann, modified by Debian

I'm wondering why this doesn't work - many people are planning their
whole life with org mode (as I'm doing as well), is any help available?