编码器错误:未定义偏移量1

我的代码有问题。我得到的错误如下:

$vf=$bn[1];

$ this→_data("县")→Post in html

此错误:

严重性:Notice

消息:Undefined offset:1

Filename: mustahik/mustahik_controller.php

Line .php数量:1108

我的控制器代码如下

$this->_data['kabupaten'] = "";
if($this->_data['kab'] != $this->_data['kabupaten']) {
    $vf = $this->_data['kab'] ;
    $cf = $vf;
    echo "kk".$cf;
} else {
    $bn = explode('.',($this->_data['kabupaten']), 3);
    $vf = $bn[1]; /// error here
    $cf = $vf;
    echo "kk".$cf;
}

My view is a below

<tr>
    <td>Kab/Kota<span class="red">*</span></td>
    <td>
        <input type="text" name="kab" value="<?php echo $survey['kabupaten'];?>">
        <select name="kabupaten" id="kabupaten" style="width: 100%;" class="select2" onchange="get_districts(this.value);" >
        </select>
        <option value="<?php echo $survey['kabupaten'];?>"></option>
    </td>
</tr>

有人知道是什么问题吗?