Undefined index: ac in

Estou resolvendo uns probleminhas para uma locadora e me deparei com o seguinte problema ,

Layout

CLIQUE AQUI PARA CADASTRAR NOVO CLIENTE

SELECIONE A LETRA E VISUALIZE OS CLIENTES CADASTRADOS.

A B C D E F G H I J K L M N O P Q R S T U V X Y W Z

SELECIONE O CLIENTE PARA VISUALIZAR OS DADOS CODIGO

Notice: Underfined Index ac in c:\xamp\htdocs\...\listar.php

codigo

<table width="88%" border="1" align="center" cellpadding="4" cellspacing="0" bordercolor="#000099">
 <tr>
 <td class="style3"><div align="center" class="style15"><a href="listar.php?ac=a">A</a></div></td>
 <td class="style3"><div align="center" class="style15"><a href="listar.php?ac=b">B</a></div></td>
 <td class="style3"><div align="center" class="style15"><a href="listar.php?ac=c">C</a></div></td>
 <td class="style3"><div align="center" class="style15"><a href="listar.php?ac=d">D</a></div></td>
 <td class="style3"><div align="center" class="style15"><a href="listar.php?ac=e">E</a></div></td>
 <td class="style3"><div align="center" class="style15"><a href="listar.php?ac=f">F</a></div></td>
 <td class="style3"><div align="center" class="style15"><a href="listar.php?ac=g">G</a></div></td>
 <td class="style3"><div align="center" class="style15"><a href="listar.php?ac=h">H</a></div></td>
 <td class="style3"><div align="center" class="style15"><a href="listar.php?ac=i">I</a></div></td>
 <td class="style3"><div align="center" class="style15"><a href="listar.php?ac=j">J</a></div></td>
 <td class="style3"><div align="center" class="style15"><a href="listar.php?ac=k">K</a></div></td>
 <td class="style3"><div align="center" class="style15"><a href="listar.php?ac=l">L</a></div></td>
 <td class="style3"><div align="center" class="style15"><a href="listar.php?ac=m">M</a></div></td>
 <td class="style3"><div align="center" class="style15"><a href="listar.php?ac=n">N</a></div></td>
 <td class="style3"><div align="center" class="style15"><a href="listar.php?ac=o">O</a></div></td>
 <td class="style3"><div align="center" class="style15"><a href="listar.php?ac=p">P</a></div></td>
 <td class="style3"><div align="center" class="style15"><a href="listar.php?ac=q">Q</a></div></td>
 <td class="style3"><div align="center" class="style15"><a href="listar.php?ac=r">R</a></div></td>
 <td class="style3"><div align="center" class="style15"><a href="listar.php?ac=s">S</a></div></td>
 <td class="style3"><div align="center" class="style15"><a href="listar.php?ac=t">T</a></div></td>
 <td class="style3"><div align="center" class="style15"><a href="listar.php?ac=u">U</a></div></td>
 <td class="style3"><div align="center" class="style15"><a href="listar.php?ac=v">V</a></div></td>
 <td class="style3"><div align="center" class="style15"><a href="listar.php?ac=x">X</a></div></td>
 <td class="style3"><div align="center" class="style15"><a href="listar.php?ac=y">Y</a></div></td>
 <td class="style3"><div align="center" class="style15"><a href="listar.php?ac=w">W</a></div></td>
 <td class="style3"><div align="center" class="style15"><a href="listar.php?ac=z">Z</a></div></td>
 </tr>
 </table>
 <span class="style3"><br>
 </span>
 <table width="70%" border="1" align="center" cellpadding="4" cellspacing="0" bordercolor="#000099">
 <tr>
 <td bgcolor="#3366CC"><div align="center" class="style14"><span class="style11">SELECIONE O CLIENTE PARA VISUALIZAR OS DADOS</span></div></td>
 <td bgcolor="#3366CC"><div align="center" class="style14"><span class="style11">CODIGO</span></div></td>
 </tr>
<?php

 $a = $_GET['ac'];