HtmlEditorExtender character count via Javascript not working

I'm using the Ajax HtmlEditorExtender and I need to count the characters of a textbox.

I use a javascript code to count the characters (remaining & left) and it works fine with regular textboxes in asp.net. But if i use it for a textbox that is related to HtmlEditorExtender. The Javascript code does not work-- (The function is not called).

My code looks like this:

Javascript code:

<script type="text/javascript">
function textCounter(field, countfield) {
    if (field.value.length > 100)
            field.value = field.value.substring(0, 100);
    else
            countfield.value = 100 - field.value.length;
 }</script>

default.aspx page:

<input readonly="readonly"  type="text"   id="remLen" size="3" maxlength="3" value="160" />
<asp:TextBox    ID="txtSlideTexts"  TextMode="MultiLine"   Columns="50"    Rows="8"      Runat="server" /><ajaxToolkit:HtmlEditorExtender
        ID="hee"
        TargetControlID="txtSlideTexts"
        Runat="server"   DisplaySourceTab="true"><Toolbar> <ajaxToolkit:Undo /><ajaxToolkit:Redo /><ajaxToolkit:Bold /><ajaxToolkit:Italic /><ajaxToolkit:Underline /><ajaxToolkit:UnLink /><ajaxToolkit:RemoveFormat /><ajaxToolkit:Cut /><ajaxToolkit:Copy /><ajaxToolkit:Paste /></Toolbar></ajaxToolkit:HtmlEditorExtender>

and finally the Default.aspx.cs code:

protected void Page_Load(object sender, EventArgs e)
{
    txtSlideTexts.Attributes.Add("onkeyup", "return textCounter(this,  remLen);");
    txtSlideTexts.Attributes.Add("onkeydown", "return textCounter(this,  remLen);");
}

any suggestions?