Webpart i ASP.NET 2.0

Jeg har en UserControl, jeg gerne vil lave til en webpart. Jeg har sat den til at bruge interfacet System.Web.UI.WebControls.WebParts.IWebPart og tilføjet de properties, der kræves. Men alligevel får jeg følgende fejl:

Make sure that the class defined in this code file matches the 'inherits' attribute, and that it extends the correct base class (e.g. Page or UserControl).

Min ascx-side ser sådan ud:

<%@ Control Language="C#" AutoEventWireup="true" CodeFile="Lommeregner.ascx.cs" Inherits="WebPartsNS.Lommeregner"  %>
<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
<br />
+<br />
<asp:TextBox ID="TextBox2" runat="server"></asp:TextBox><br />
<asp:Button ID="Button1" runat="server" OnClick="Button1_Click" Text="Udregn" />
<br />
<br />
<asp:TextBox ID="TextBox3" runat="server"></asp:TextBox>

Min codebehind-fil ser således ud:

namespace WebPartsNS
{
    using System;
    using System.Data;
    using System.Configuration;
    using System.Collections;
    using System.Web;
    using System.Web.Security;
    using System.Web.UI;
    using System.Web.UI.WebControls;
    using System.Web.UI.WebControls.WebParts;
    using System.Web.UI.HtmlControls;

    public partial class Lommeregner : System.Web.UI.WebControls.WebParts.IWebPart
    {
        private String _catalogImageUrl = string.Empty;
        private String _description = string.Empty;
        private String _subTitle = "[0]";
        private String _title = "Super cool user control ";

        public String CatalogIconImageUrl
        {
            get { return _catalogImageUrl; }
            set { _catalogImageUrl = value; }
        }
        public String Description
        {
            get { return _description; }
            set { _description = value; }
        }
        public String Subtitle
        {
            get { return string.Empty; }
            set { ; }
        }
        public String Title
        {
            get { return _title; }
            set { _title = value; }
        }
        public String TitleIconImageUrl
        {
            get { return string.Empty; }
            set { ; }
        }
        public String TitleUrl
        {
            get { return string.Empty; }
            set { ; }
        }

        protected void Page_Load(object sender, EventArgs e)
        {

        }
        protected void Button1_Click(object sender, EventArgs e)
        {
            int num1 = Convert.ToInt32(TextBox1.Text);
            int num2 = Convert.ToInt32(TextBox2.Text);

            int sum = num1 + num2;
            TextBox3.Text = sum.ToString();
        }
    }
}

Hvad er problemet? Jeg kan ikke hitte ud af det...