error while trying to show "Monthly purchase by a category" after clicking on the graph

[Mihai Satmarean]

Binary package hint: openerp-server

I am using mintlinux 10, and tried to open from Firefox.

Purchase->purchase dashboard, there is a graph on the left side,
 when cicking on he graph, the following error is displayed.

Traceback (most recent call last):
  File "/usr/lib/openerp-server-6.0.0-rc2/netsvc.py", line 489, in dispatch
    result = ExportService.getService(service_name).dispatch(method, auth, params)
  File "/usr/lib/openerp-server-6.0.0-rc2/service/web_services.py", line 598, in dispatch
    res = fn(db, uid, *params)
  File "/usr/lib/openerp-server-6.0.0-rc2/osv/osv.py", line 122, in wrapper
    return f(self, dbname, *args, **kwargs)
  File "/usr/lib/openerp-server-6.0.0-rc2/osv/osv.py", line 176, in execute
    res = self.execute_cr(cr, uid, obj, method, *args, **kw)
  File "/usr/lib/openerp-server-6.0.0-rc2/osv/osv.py", line 167, in execute_cr
    return getattr(object, method)(cr, uid, *args, **kw)
  File "/usr/lib/openerp-server-6.0.0-rc2/osv/orm.py", line 2143, in read_group
    assert not groupby or groupby in fields, "Fields in 'groupby' must appear in the list of fields to read (perhaps it's missing in the list view?)"
AssertionError: Fields in 'groupby' must appear in the list of fields to read (perhaps it's missing in the list view?)