Breaking natbib support

[Juergen Spitzmueller]

The following entry breaks LyX's natbib support (the citation style choice 
gets disabled). And yes, this is a real example from my real work.

Regards,
Jürgen.

@Article{stk:ausstatter01,
  Author         = {s.t.k.},
  Title          = {Aus.statter [sic!]},
  Journal        = {Frankfurter Rundschau},
  Pages          = {1},
  month          = {24} # feb,
  year           = 2001,
}