OpenWorklistItem always throws exception for global variable

I have an activity with a client side event in it.
There are also 2 Process variables, one a string and one a boolean attached to the process.
Within code I am trying to open the worklistitem associated with this client side event, however every time I do so, I receive an exception stating that the boolean processinstance variable threw an exception of type system.exception (data field approved not found).
Here is the code I'm using :-

.Value = true;
  }
  catch(Exception ex)
  {
   string test = ex.Message.ToString();
   System.Diagnostics.Debug.WriteLine(test);
  }
 }

ItemSelected is taken from a listbox that I previously populated with active events for that user.
I've looked through the KB, help, tutorials etc. and can't find what's wrong.
Any help appreciated.

Colin Brown