Request.Querystring["ReturnUrl"] question

Using the WebSite template project from VS 2012. The  Register link is a hyperlink that opens Register.aspx. In the page_open of Register.aspx, the code is:
  RegisterUser.ContinueDestinationPageUrl = Request.QueryString["ReturnUrl"];
I assume that in that case the ReturnUrl is not null, because the page does redirect once you register.
I dont want to use the RegisterUser Wizard, so my Register.aspx page has:
  string  retUrl = Request.QueryString["ReturnUrl"].ToString();
But the ReturnUrl is always null. Is this because the link that opens the page is from a Master page? I am using the same code for the Master page link: 
<asp:LoginView ID="LoginView1" runat="server" ViewStateMode="Disabled"> <AnonymousTemplate> <ul> <li><a id="registerLink" runat="server" href="~/Ac ...