I've installed the [captcha plugin](https://www.nuget.org/packages/Captcha/) and followed the setup instructions. The captcha renders perfectly on the page but when submit the form, the ModelState.IsValid is always true, no matter what I enter. Obviously, if captcha won't validate then it's not much good to me.

Here's my controller:

```
        [HttpPost]
    [CaptchaValidation("CaptchaCode", "SampleCaptcha", "Incorrect CAPTCHA code!")]
    public ActionResult Register(AccountModel model)
    {
        if (!ModelState.IsValid)
        {
            // TODO: Captcha validation failed, show error message
        }
        else
        {
            // TODO: Captcha validation passed, proceed with protected action
        }

        return View();
    }

```

And here's my view:

```
<link href="@BotDetect.Web.CaptchaUrls.Absolute.LayoutStyleSheetUrl" rel="stylesheet" type="text/css" />

<form class="form-horizointal" action="@Url.Action("Register", "Account")" method="POST">
    <div class="form-group">
        @Html.LabelFor(m => m.FirstName, new { @class = "col-sm-2 control-label" })
        @Html.TextBoxFor(m => m.FirstName, new { placeholder = "First Name"})
        @Html.LabelFor(m => m.LastName, new { @class = "control-label", placeholder = "Last Name" })
        @Html.TextBoxFor(m => m.LastName, new { placeholder = "Last Name" })
    </div>
    <div class="form-group">
        @Html.LabelFor(m => m.Email, new {@class = "col-sm-2 control-label", placeholder = "Email"})
        @Html.TextBoxFor(m => m.Email, new {placeholder = "Email"})
    </div>
    <div class="form-group">
        @{ MvcCaptcha sampleCaptcha = new MvcCaptcha("SampleCaptcha"); }
        @Html.Captcha(sampleCaptcha)
        @Html.TextBox("CaptchaCode")
    </div>
    <div class="form-group">
        captcha goes here
    </div>
    <div class="form-group">
        <input type="submit" class="btn btn-default" value="Register"/>
    </div>
</form>

```

Does anyone have any idea why this isn't working for me? Thanks for your help!