create user wizard control ignores checking captcha

Hi friends ,

I have to check captcha code if wrong, prevent user register, but create user wizard ingores it, why?

protected void CreateUserWizard1_NextButtonClick(object sender, WizardNavigationEventArgs e)
    {
        TextBox captchatxt = (TextBox)CreateUserWizardStep1.ContentTemplateContainer.FindControl("TextBox1");
        Literal wrongcaptcha = (Literal)CreateUserWizardStep1.ContentTemplateContainer.FindControl("WrongCaptcha");
        if (Session["Captcha"].ToString() != captchatxt.Text)
        {
            wrongcaptcha.Text = "Wrong Captcha";
            captchatxt.Text = "";
        }

    }

if wrong or right captcha user will register.

what I have to do, plz help, thanks