Fatal error: Can’t use function return value in write context line 142

码：

<div>
    <div class="">
        <input type="text" id="select_shelves" name="select_shelves" class="shelves_select_and_buttons" />
        <div id="shelves_menu" >
            <ul>
                <li id="li_" onclick="printValue();">option5 <= line 142 </li>
            </ul>
        </div>
    </div>
    <div class="button">
        <input type="button" onclick="dropShelves();" id="Shelves_select_button" name="Shelves_select_button" value="" class="grey_button"/>
    </div>
</div>