How do I fix a compilation error?

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C#

.NET

.NET4

I'm currently receiving an error that reads:
Element 'X' is not a know element. This can occur if there is a compilation error in the Web site, or the web.config file is missing.

I should note that this is happening within a webform I've added which also has a default masterpage. The file type is .aspx the code below shows how I've configured the web form excluding the actual content.

<%@ Page Title="" Language="C#" MasterPageFile="~/Default.Master" AutoEventWireup="true" CodeBehind="SearchPage.aspx.cs"
     Inherits="Project.Search_Page.SearchPage" %>

<%@ MasterType VirtualPath="~/Default.master" %>
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="AjaxToolkit" %>
<%@ Register Src="~/Controls/InfoMessage.ascx" TagName="InfoMsg" TagPrefix="CustCntl" %>
<%@ Register Src="~/Controls/ContentBox.ascx" TagName="ContentBox" TagPrefix="CustCntl" %>
<%@ Register Src="~/Controls/ModalBox.ascx" TagName="ModalBox" TagPrefix="CustCntl" %>
<%@ Register Src="~/Controls/GridViewHeader.ascx" TagName="GVHeader" TagPrefix="CustCntl" %>
<%@ Register Assembly="Custom.Web.UI" Namespace="Custom.Web.UI" TagPrefix="Custom" %>

I don't know what is causing this issue because I have the exact same code, at least from MasterType VirtualPath = "~/Default..... down to everything below. I've also checked my web config file and didn't find any issues there.

Any ideas on how to approach/fix this error?