onclick - show hide

TypeError: document.getElementById(...) is null
hvad gør jeg forkert her?


<h3 onclick='showTable("<%=objProd("ProductNumber")%>");' id='open<%=objProd("ProductNumber")%>'><%=objProd("ProductNumber")%></h3>
<h3 onclick='hideTable("<%=objProd("ProductNumber")%>");' id='close<%=objProd("ProductNumber")%>' class='closed'><%=objProd("ProductNumber")%></h3>

<%
response.write "<table class='prodtable' id='tabel"&objProd2("ProductTitel")&"'>"
%>

<script language="javascript" type="text/javascript">
  function showTable(prodnumber) {
    //$("#tabel" + prodnumber).show();
    //$("#close" + prodnumber).show();
    //$("#open" + prodnumber).hide();
    //$("#tabel" + prodnumber).css("display", "block");
    //$("#close" + prodnumber).css("display", "block");
    //$("#open" + prodnumber).css("display", "none");
    document.getElementById("tabel" + prodnumber).style.display = "block";
    document.getElementById("close" + prodnumber).style.display = "block";
    document.getElementById("open" + prodnumber).style.display = "none";
  }
  function hideTable(prodnumber) {
    //$("#tabel" + prodnumber).hide();
    //$("#close" + prodnumber).hide();
    //$("#open" + prodnumber).show();
    //$("#tabel" + prodnumber).css("display", "none");
    //$("#close" + prodnumber).css("display", "none");
    //$("#open" + prodnumber).css("display", "block");
    document.getElementById("tabel" + prodnumber).style.display = "none";
    document.getElementById("close" + prodnumber).style.display = "none";
    document.getElementById("open" + prodnumber).style.display = "block";
  }
</script>