Hi everyone, 

I have this frequent error (but not always), when I'm in the org agenda view trying to clock in the task under cursor. Here's a stack trace:

Debugger entered--Lisp error: (wrong-type-argument stringp nil)

  org-agenda-clock-in(nil)

  funcall-interactively(org-agenda-clock-in nil)

  call-interactively(org-agenda-clock-in nil nil)

  command-execute(org-agenda-clock-in)

Note, every time this occurs, I can refresh the agenda view, then clock in with no problems.  But this refreshing gets tiring..