org-clock-in fails sometimes with prefix arg

[Bernt Hansen <bernt@norang.ca>]

Hi Carsten,

I've run into this a few times today.  I don't have any useful insight
into how this occurs yet but I thought I'd mention it anyway.

My current org-clock-history looks like this:

,----[ C-h v org-clock-history RET]
| org-clock-history is a variable defined in `org-clock.el'.
| Its value is 
| (#<marker at 5990 in norang.org> #<marker at 42510 in norang.org> #<marker at 8870 in csp.org> #<marker at 6269 in org.org> #<marker at 5145 in ciss.org> #<marker at 339 in org> #<marker at 1 in org> #<marker at 10546 in csp.org> #<marker at 9659 in csp.org> #<marker at 10794 in csp.org>)
| 
| 
| 
| Documentation:
| List of marker pointing to recent clocked tasks.
`----

Somehow I am getting markers to read-only buffers in there 

 eg.    (#<marker at 1 in org>)

and this breaks C-u M-x org-clock-in which triggers the following error
in org-clock-select-task

,----
| save-restriction: Buffer is read-only: #<buffer org>
`----

I'm not sure how I got these markers in the list

org-clock-select-task fails when it hits the read-only buffer.  This org
buffer happens to be a directory (in dired I believe) of my ~/git/org
directory.

As a workaround for now I'm just manually clearing the variable with

  (setq org-clock-history nil)

to remove the markers and make this work again.

Regards,
Bernt