How to style input with radio addon for error messages - twitter bootstrap

I am trying to style input field with radio addon for error message (.help-block). it works fine without radio addons . But with addon, text jump to new line under input field.

it is look like this

HTML

<div class="input-group has-error">
  <span class="input-group-addon">
    <input type="radio" name="answer" value="a">
  </span>
  <input type="text" class="form-control" placeholder="Your answer here..." name="a" id="a">
  <span class="help-block">The a field is required</span>
</div>

So what should I do that provided by bootstrap 3.