How to Implement captcha in Asp.net issue..

Hi Mudassir bro The captcha is working fine..but when we reload the page and click the verify button without entering enything..it's throwing error. Also it's throwing the error when we enter the code wrong 2 times.. It's throwing the error as

Object reference not set to an instance of an object.

Please resolve the issue..

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Line 15:protected void btnVerify_Click(object sender, EventArgs e)
Line 15:     {
Line 16:         Captcha1.ValidateCaptcha(txtCaptcha.Text.Trim());
Line 17: 
Line 18:         if (Captcha1.UserValidated)