Problem retrieving an access token with code returned

I am having a problem retrieving the user's access token after he/she has authorized my Facebook application to access their information and post for them, etc... Facebook returns a code query string to my website, so I can receive the access token for the user. I use the following code to get the access code.

    string AppKey = "[REMOVED]";
    string AppSecret = "[REMOVED]";
    var oAuth = new Facebook.FacebookOAuthClient();
    oAuth.AppId = AppKey;
    oAuth.AppSecret = AppSecret;
    oAuth.RedirectUri = new Uri("http://www.mywebsite.com");
    Label3.Text = Request.QueryString["code"];
    try
    {
        var accessToken = oAuth.ExchangeCodeForAccessToken(Request.QueryString["code"]);
        string accessTokenString = accessToken.ToString();
        HttpCookie aCookie = new HttpCookie("MyWebsite_FBAccessToken");
        aCookie.Value = accessTokenString;
        Response.Cookies.Add(aCookie);
        Response.Redirect("~/Process/ProcessToken.aspx");
    }
    catch (Facebook.FacebookOAuthException error)
    {
     开发者_如何学运维   Label2.Text = error.Message;
    }

My code gets held up here:

var accessToken = oAuth.ExchangeCodeForAccessToken(Request.QueryString["code"]);

And I receive the following error.

(OAuthException) Error validating verification code.

Does this seem like there is a problem with my code, or does it look like there may be a setting problem with my Facebook application? I know my App ID and Secret are correct.