set text of a htmleditorextender

Hi,

I'm trying to set the value of a textbox with htmleditorextender with js.

I have this code:

function ValidatePayment() {
    var hiba = 0;
    var eloleg = document.forms["adv_paym"]["txtElolegosszeg"].value;
    if (eloleg == null || eloleg == "" || eloleg == "Kötelező mező!") {
        document.getElementById("txtElolegosszeg").setAttribute("class", "errortextbox");
        document.getElementById("txtElolegosszeg").value = "Kötelező mező!";
        hiba = hiba + 1;
    }
    var indoklas = document.forms["adv_paym"]["txtIndoklas"].value;
    if (indoklas == null || indoklas == "" || indoklas == "Kötelező mező!") {
        document.getElementById("txtIndoklas").setAttribute("class", "errortextbox");
        document.getElementById('ctl00_ContentPlaceHolder1_txtIndoklas_HtmlEditorExtender_ExtenderContentEditable').innerHTML = "Kötelező mező!"
        hiba = hiba + 1;
    }
    if (hiba > 0) { return false; }
}

But it doesn't work.

And curiously, it makes a postback, despite the fact, that the "hiba" variable is greater then 0.

If I remove the line with the htmleditorextender, it don't make a postback.

And here is the markup of the textbox:

<asp:TextBox ID="txtIndoklas" runat="server" CssClass="mytextbox2" 
                        Height="200px" TextMode="MultiLine" Width="500px"></asp:TextBox><asp:HtmlEditorExtender ID="txtIndoklas_HtmlEditorExtender" runat="server" 
                        Enabled="True" TargetControlID="txtIndoklas" EnableSanitization="false"></asp:HtmlEditorExtender>