게시판 검색 입력 시 "undefined할 게시물을 하나 이상 선택하세요."

<!-- 게시판 검색 시작 { -->
<fieldset id="bo_sch">
    <legend>게시물 검색</legend>

    <form name="fsearch" method="get">
    <input type="hidden" name="bo_table" value="<?php echo $bo_table ?>">
    <input type="hidden" name="sca" value="<?php echo $sca ?>">
    <input type="hidden" name="sop" value="and">
    <label for="sfl" class="sound_only">검색대상</label>
    <select name="sfl" id="sfl">
        <option value="wr_subject"<?php echo get_selected($sfl, 'wr_subject', true); ?>>제목</option>
        <option value="wr_content"<?php echo get_selected($sfl, 'wr_content'); ?>>내용</option>
        <option value="wr_subject||wr_content"<?php echo get_selected($sfl, 'wr_subject||wr_content'); ?>>제목+내용</option>
        <option value="mb_id,1"<?php echo get_selected($sfl, 'mb_id,1'); ?>>회원아이디</option>
        <option value="mb_id,0"<?php echo get_selected($sfl, 'mb_id,0'); ?>>회원아이디(코)</option>
        <option value="wr_name,1"<?php echo get_selected($sfl, 'wr_name,1'); ?>>글쓴이</option>
        <option value="wr_name,0"<?php echo get_selected($sfl, 'wr_name,0'); ?>>글쓴이(코)</option>
    </select>
    <label for="stx" class="sound_only">검색어<strong class="sound_only"> 필수</strong></label>
    <input type="text" name="stx" value="<?php echo stripslashes($stx) ?>" required id="stx" class="frm_input required" size="15" maxlength="20">
    <input type="submit" value="검색" class="btn_submit">
    </form>
</fieldset>
<!-- } 게시판 검색 끝 -->