[jQuery] handler is undefined error

[macsig]

Hello guys,
I'm trying to create an horizontal accordion using the code below:

$(document).ready(function(){
    lastBlock = $("#a1");
    maxWidth = 210;
    minWidth = 75;

    $(".accordion_trigger").hover(function(){
        $(lastBlock).animate({width: minWidth+"px"}, { queue:false,
duration:400 });
        $(this).animate({width: maxWidth+"px"}, { queue:false, duration:
400});
        lastBlock = this;
      }
    );
});

with the following HTML:

<ul class="accordion">
  <li>
    <span id="a1" class="accordion_trigger">
      <img src="images/free_thumb.jpg" />
      <p>
        <strong>Freebies</strong><br/>
        Download free files to make your job easier.
      </p>
    </span>
  </li>
  <li>
    <span class="accordion_trigger">
       <img src="images/tut_thumb.jpg" />
       <p>
         <strong>Tutorials</strong><br/>
         Tips and tricks to help you
         keep up with the latest technology.
       </p>
    </span>
  </li>
  <li>
    <span class="accordion_trigger">
      <img src="images/inspire_thumb.jpg" />
      <p>
        <strong>Inspiration</strong><br/>
        Get inspired by what other designers are doing.
      </p>
    </span>
  </li>
</ul>


The effect works well but I get this error:

handler is undefined
http://localhost:3000/javascripts/jquery.js?1236564794
Line 25

Any idea about the reason I get it?

I'm using some other scripts and plug-ins. Can it be a conflict issue?

Thanks and have a nice day