Rails - Apr 2006 - undefined method `stringify_keys!'' for "Tue Apr 18 11:26:00 India Standard Time 2006":String

hi all 
i am submitting form and i take valu from two text fields.
my controller code is like that
def saleinsert
   @verkaufsliste = Verkaufsliste.create(params[:bestandsliste])
   @listes = Verkaufsliste.create(params[:verkaufsliste.to_s])
   @verkaufsliste=Verkaufsliste.create(@listes.vkdatum.to_s)
if @verkaufsliste.save 
	      redirect_to :action => ''list''
else
	      render :action => ''sale''
end
end