Custom crawler function gives strange output only in my custom theme

I am using the following function to make the nav a select list

<nav class="page-name">
        <?php
            class Walker_Nav_Menu_Dropdown extends Walker_Nav_Menu{
                function start_lvl(&$output, $depth){
                    $indent = str_repeat("\t", $depth); // don't output children opening tag (<code><ul></code>)
                }

                function end_lvl(&$output, $depth){
                    $indent = str_repeat("\t", $depth); // don't output children closing tag
                }

                function start_el(&$output, $item, $depth, $args){ // add spacing to the title based on the depth
                    $item->title = str_repeat("?", $depth * 2).$item->title;

                    parent::start_el(&$output, $item, $depth, $args);

                    // no point redefining this method too, we just replace the li tag...
                    $output = str_replace('<li', '<option', $output);
                }

                function end_el(&$output, $item, $depth){
                    $output .= "</option>\n"; // replace closing </li> with the option tag
                }
            }

            wp_nav_menu(array(
                'theme_location' => 'primary', // your theme location here
                'walker'         => new Walker_Nav_Menu_Dropdown(),
                'items_wrap'     => '<select>%3$s</select>',
            ));
        ?>
    </nav>

In twentyeleven theme it gives the right html:

<nav class="page-name">
        <div class="menu-nav-menu-container">
            <select>
                <option id="menu-item-59" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-59">Sample Page </option>
                <option id="menu-item-60" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-60">??News</option>
                <option id="menu-item-65" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-65">About   </option>
                <option id="menu-item-62" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-62">??Contact Us</option>
                <option id="menu-item-63" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-63">??Our Location</option>
                <option id="menu-item-61" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-61">History</option>
                <option id="menu-item-64" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-64">Links</option>
            </select>
        </div>
    </nav>

However in my theme it gives me this:

<nav class="page-name">
        <div class="menu">
            <ul>
                <option id="menu-item-54" class="menu-item-54"><a></a></option>
                <option id="menu-item-24" class="menu-item-24"><a></a></option>
                <option id="menu-item-30" class="menu-item-30"><a></a></option>
                <option id="menu-item-18" class="menu-item-18"><a></a></option>
                <option id="menu-item-22" class="menu-item-22"><a></a></option>
                <option id="menu-item-26" class="menu-item-26"><a></a></option>
                <option id="menu-item-13" class="menu-item-13"><a></a></option>
                <option id="menu-item-28" class="menu-item-28"><a></a></option>
                <option id="menu-item-15" class="menu-item-15"><a></a></option>
                <option id="menu-item-2" class="menu-item-2"><a></a></option>
                <option id="menu-item-20" class="menu-item-20"><a></a></option>
            </ul>
        </div>
    </nav>

I both themes I use the same primary menu, and in both the normal menu crawler works fine.

What could be causing this?