Problems with Instance Variable in Controllenr

I've got problems with an instance variable in a controller.
the varible is initialized and has some entries which i give out in
the
second view(create). but in the third view (export) I become back this
message:
...............ActionView::Template::Error (undefined method `length'
for nil:NilClass):
    1: <%=@month_issues.class%>
    2: <%=@month_issues.length%>
    3: <%@month_issues.each do |mi|%>
    4: <%= mi.text %>
    5: <%end%>
  plugins/abrechnung/app/views/abrechnung/export.html.erb:2:in
`_plugins_abrechnung_app_views_abrech
nung_export_html_erb___625827306_55588872'.............
But in abrechnung_controller the vaiable is initialised as Array with
some antries, how it can be nil?

Attachments:
http://www.ruby-forum.com/attachment/10248/abrechnung_controller.rb
http://www.ruby-forum.com/attachment/10249/month_issue.rb
http://www.ruby-forum.com/attachment/10250/export.html.erb
http://www.ruby-forum.com/attachment/10251/routes.rb

You realize instance variables are not carried forward from one action to another? They have to be set up on each request.