C# - Session Is Null When Inherit From System.Web.UI.Page?

I want to extend the System.Web.UI.Page-class with some extra stuff.
In the ctor I need the value of a session-variable.

The problem is that the Session-object is null...

public class ExtendedPage : System.Web.UI.Page {
protected foo;
public ExtendedPage() {
this.foo = (int)HttpContext.Current.Session["foo"]; // NullReferenceException
}
}

If I move the part with the session-object into the Load-Event everything works fine...

public class ExtendedPage : System.Web.UI.Page {
protected foo;
public ExtendedPage() {
this.Load += new EventHandler(ExtendedPage_Load);
}
void ExtendedPage_Load(object sender, EventArgs e) {
this.foo = (int)HttpContext.Current.Session["foo"];
}
}

Why is the Session-object null in the first case??