Error: Element 'ContentPlaceholder' is not a known element.

Using Visual Web Developer 2010 express, I created an empty Web application.  The next thing I did was create a master page.  Upon the page opening, the <asp:ContentPlaceHolder...> tag was flagged and when I mouse over it, the message:
Element 'ContentPlaceholder' is not a known element.  This can occur if there is a compilation error in the Web site or the web.config file is missing.
The web.config file is there, albeit with not much in it.
The Intellisense does not recognize any <asp: tags either.
Any suggestions how to fix this?
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