Trouble getting value from Checkbox in Firefox

I have the following checkbox

<form id="Form1" runat="server">
<p class="hint grey"><asp:checkbox runat="server" ID="chkBox"/><asp:literal runat="server" ID="warningTxt"></asp:literal></p>

<div class="clear"></div>

<br/>
<table>
<tr><td>
<asp:ImageButton ID="btnSubmit" 
                    runat="server" 
                    Text="Agree" 
                    ToolTip="Agree"  
                    UseSubmitBehavior="true"
                    OnClick="btnSubmit_Click" 
                    CausesValidation="true"
                    ImageUrl="~/img/buttons/agree-button.gif" />
                    </td><td>
                    <a href="#" onclick="window.print();return false;"><img src="img/buttons/print-button.gif"/></a>
                    </td></tr></table>
                </form>

And in code behind

protected void btnSubmit_Click(object sender, EventArgs e)
{
    ErrorMessage.Text = String.Empty;
    DateTime currentDate = DateTime.Now;
    if (Page.IsValid)
    {

        if (chkBox.Checked)
        {
            Response.Redirect("Default.aspx", false);
        }
        else
        {
            ErrorMessage.Text = "Tick the alert statement";
         开发者_JS百科   ErrorMessage.CssClass = "error";
            //errorTable.Visible = true;
        }

    }
}

Now for some reason in Firefox, when I tick the box and click on submit I get the error message "Tick the alert statement", but its fine in Chrome. So just why firefox isnt picking up the value in checkbox correct ?

Try removing the CausesValidation="true" attribute. Its the only thing I can see that could be causing problems.