Parser Error Message: The file '/Site1.Master' does not exist --- but it does

Parser Error Message: The file '/Site1.Master' does not exist --- but it does

Seems like an easy-to-fix error, but I've been banging my head on this for
a while. App works fine in the VS built-in webserver, but not after moving
it to the dev server with IIS7.
The default.aspx file refers to the master page in the page directive...
<%@ Page Title="" Language="C#" MasterPageFile="~/Site1.Master"
AutoEventWireup="true" CodeBehind="Default.aspx.cs"
Inherits="MyApp.Default" %>
Site1.Master is in the site's root directory, along with default.aspx. The
folder permissions are read to Everyone at this point.
This is an intranet site so IIS authorization is anonymous disabled,
windows auth enabled, impersonation enabled. App pool is .NET v4, managed
pipeline integrated.
I've looked through this site and others for answers and while others have
had the same problem, it was for different reasons.
Thanks.