why XMLHTTP status==500 and have not responsetext?

Question

i have xmlhttp status==500 and not responsetext,my showtable.php echo a table with different content in several conditions.

$("#host-type-list").click(function(){
   var hostvalue=$("#selected-host").text();
   if(window.XMLHttpRequest) {
      xmlhttp=new XMLHttpRequest();
   }
   else {
      xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
   }
   xmlhttp.onreadystatechange=function() {
     if (xmlhttp.readyState==4) {
        document.getElementById("tbldescription").innerHTML = xmlhttp.responsetext;
     }
   }
   xmlhttp.open("GET","showtable.php?req="+hostvalue,true);
   xmlhttp.send();
});