ASP.NET Login Control with reCAPTCHA
===

I have put a reCAPTCHA in my login controller. On the OnLoggingIn event, I'm calling the function to validate reCAPTCHA.

    <asp:Login ID="LoginUser" runat="server" EnableViewState="false" RenderOuterTable="false"
    OnLoginError="loginFailed" OnLoggingIn="checkCaptcha">

and this is my codebehind function to validate captcha:

    protected void checkCaptcha(object sender, LoginCancelEventArgs e)
    {
    Recaptcha.RecaptchaControl captcha = (Recaptcha.RecaptchaControl)LoginUser.FindControl("recaptcha");
    captcha.Validate();
    if (!captcha.IsValid)
    {
        e.Cancel = true;
    }
    }

but even if captcha text is not correct, user is able to log in. How can I prevent user from logging in when captcha is not valid?