Rails - May 2006 - Problems validating form with collection_select

I have the following models:

class Comandancia < ActiveRecord::Base
  has_many :elementos
end

class Elemento < ActiveRecord::Base
  belongs_to :comandancia
  validates_presence_of :nombre, :apellido_paterno, :apellido_materno,
:comandancia
end

In the view for New Elemento I''m using
<%= start_form_tag :action => ''create'' %>
  <%= render :partial => ''form'' %>
  <%= collection_select(''elemento'',
''comandancia_id'', @comandancia, ''id'',
''nombre'')%>
  <%= submit_tag "Create" %>
<%= end_form_tag %>

When I create a New Elemento and provide values for all the required fields,
everything is OK, the collection_select works just fine and stores the
proper value in the Elemento table.

BUT, when I don''t provide one of the required fields so that the model
doesn''t validate, I get the following error:

Showing app/views/adminelemento/new.rhtml where line #5 raised:
You have a nil object when you didn''t expect it!
You might have expected an instance of Array.
The error occured while evaluating nil.inject

And it points the error to the collection_select line in new.rhtml. Why
collection_select becomes a problem if the validation is failing because of
the OTHER fields?? How can I avoid this error?

My _form.rhtml is as follows:
<%= error_messages_for ''elemento'' %>

<!--[form:elemento]-->
<p><label
for="elemento_nombre">Nombre</label><br/>
<%= text_field ''elemento'', ''nombre''
%></p>

<p><label for="elemento_apellido_paterno">Apellido
paterno</label><br/>
<%= text_field ''elemento'',
''apellido_paterno'' %></p>

<p><label for="elemento_apellido_materno">Apellido
materno</label><br/>
<%= text_field ''elemento'',
''apellido_materno'' %></p>

<p><label
for="elemento_direccion">Direccion</label><br/>
<%= text_area ''elemento'', ''direccion''
%></p>

<!--<p><label for="elemento_comandancia_id">Id
comandancia</label><br/>
<%= text_field ''elemento'',
''comandancia_id'' %></p> -->

<!--[eoform:elemento]-->

Sorry of this post is too long but this is puzzling me.
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