Cannot have both select and radio_button together on the same form

Hello, I'm new to Ruby on Rails.

I am creating a form for searching purpose:

<% form_for :src_cond,
      :url => {:action =>'search'} do |p| %>
     <%= p.radio_button("bd", "1") %> a
     <%= p.radio_button("bd", "2") %> b
     <%= p.select :bd2, %w[1 2 3 4],{},{:index=>nil} %>
     <%= p.submit "Search" %>
<% end %>

When I click "Search", the browser shows:

"We're sorry, but something went wrong. We've been notified about
this issue and we'll take a look at it shortly."

When I check the log, it says:

  Status: 500 Internal Server Error
  expected Array (got Hash) for param `src_cond'

I've put some "puts" statements in the controller, which indicates the
controller is not called at all.

If I have only radio_button or select in the form, the controller will
be called as expected.

I wonder how I may have both combo box and radio group at the same
time.

Many thanks!!

What are you trying to achieve with {:index=>nil} on the select
helper?
<%= p.select :bd2, %w[1 2 3 4] %> should work