asp.net <asp:Content... tab

Hey, I am new here
I have the folowing problem:
I have in the Masterpage
//.....
<asp:contentplaceholder id="ContentPlaceHolder1" runat="server">

</asp:contentplaceholder>
//.....
and in every page derived from it
<%@ Page Language="C#" MasterPageFile="~/MasterPage.master" AutoEventWireup="true" CodeFile="Page.aspx.cs" Inherits="Page" Title="Untitled Page" %>
<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
//.....

Until now nothing unusual, but in every page I cannot use the design mode
because I have the folowing error when I want to choose Design(instead of source):

Could not match 'ContentPlaceHolder1' with any ContentPlaceHolder

The project works but it's very stressfulll.....please help

Stefan