Need help fixing an error populating a select box in the insert page but works in the editpage

Two Models Country and Bank. The below form is used for insert and edit page
works fine . f.collection_select works fine for edit page but throws error
for insert page. I think error it is trying to populate the selected value
in the combo box but country id is not available. Can you help me fixing the
issue? What is prompt used for?

Error

undefined method `map' for nil:NilClass

Extracted source (around line #23):

20: <%= f.label :country_id %><br />
21:
22:
23: <%= f.collection_select :country_id, @countries, :id, :name,
:prompt => "Test" %>

My guess is that @countries is nil (the clue is in the error
message). Are you setting that up in the controller action for
insert?

Colin