Replace_html adding observe_field gives non-fatal error

Rails
1

Using this function:

render :update do |page|
  page.replace_html 'state_options', :partial => 'state_options'
  page.replace_html 'city_options', :partial => 'city_options'
end

on this partial:

<%= options_from_collection_for_select(@states, :id, :name) %> <%= observe_field "state_id", :url => {snip!} :with => "state_id", :update => "city_id" %>

generates an “Illegal Character” error in the Firebug java console.
However, my application still runs fine. Without Firebug, the user
would never know. This seems to be a result of dynamically adding a
field observer. Is there any reason I should be concerned about this?