Google Recaptcha验证ASP.NET

Question

protected void CreateUserWizard1_CreatedUser(object sender, EventArgs e)
{    
    Recaptcha.RecaptchaControl mycaptcha = (Recaptcha.RecaptchaControl)CreateUserWizard1.CreateUserStep.ContentTemplateContainer.FindControl("RecaptchaControl1");
    mycaptcha.Validate();
    Boolean answer = mycaptcha.IsValid;
    if (answer == false)
    {
        ((CancelEventArgs)e).Cancel = true;
    }
}

上面是我的验证Google Recapture的代码，我在Visual Studio上使用CreateUserWizard表单。

代码没问题，直到我运行它并出现错误

An exception of type 'System.NullReferenceException' occurred in App_Web_whsxk5eb.dll but was not handled in user code

Additional information: Object reference not set to an instance of an object.

代码行mycaptcha.Validate();

我不确定如何修复它。