无法将属性“innerHTML”设置为空

我正在使用ajax来过滤专业列表中的职业。但它问我这个错误：无法设置空的属性"innerHTML"。ajax 代码是：

 <script type="text/javascript">
var req;
function filtro_pesquisa(valor) {   
if(window.XMLHttpRequest) 
{ 
req = new XMLHttpRequest(); 
} 
else if(window.ActiveXObject) 
{ 
req = new ActiveXObject("Microsoft.XMLHTTP"); 
}  
var url = "pesquisa.php?valor="+valor;   
req.open("Get", url, true);   
req.onreadystatechange = function() {   
if(req.readyState == 1) 
{ document.getElementById('col-md-4').innerHTML = 'A procurar...'; 
}   
if(req.readyState == 4 && req.status == 200) 
{
document.getElementById('col-md-4').innerHTML = '';
var resposta = req.responseText;   
document.getElementById('col-md-4').innerHTML = resposta; 
} 
} 
req.send(null); 
}
</script>

研究的形式是：

 <form class="navbar-form" role="search">
        <div class="input-group" style="margin-bottom: 10px;">
                     <h5 style="color:black;"> Encontre um profissional</h5>
                        <div class="input-group">
                        <input type="text" class="form-control" placeholder="Profissão" name="profissao" id="profissao" oninput="filtro_pesquisa(this.value);" autocomplete="off">
                        <span class="input-group-btn">
                         <button class="btn btn-default" type="button">Pesquisar</button>
                        </span>         
        </div>
        </form>

我仍然有 pesquisa.php 文件和"col-md-4"div 类中的列表代码。当我尝试进行搜索时，我出现了一个错误。感谢您的帮助。