Why Page_Error can't catch an HttpRequestValidationException?

[James Zhou]

I have the following code in a Page_Error handler :

Exception ex =Server.GetLastError();
Response.Write(ex.Message);
Response.Write(ex.StackTrace);
Server.ClearError();

But it doesn't work when an HttpRequestValidationException occurs(I input a
html tag into the TextBox control in the page and submit it).I have attach
the Page.Error event to this handler--the Page_Error subroutine.the same
code in the Application_Error works well.Why?