Double encoded ampersands in form actions

My form tag looks like:
<%= form_tag :controller => “admin”, :action => ‘update_preset’, :id =>
@preset, :popup => params[:popup], :editor => params[:editor] %>

which I want to be rendered something like:

but instead I get double encoded ampersands in the url:

This only seems to be a problem with the url_for_options within form
tags. Any suggestions?