Link linkbutton in method on page to linkbutton in master page

I'm attempting to make a cart system for my C# based website using ASP.NET.

Currently I've created a method which updates a linkbutton which I have in the master page.

When I try and reference the linkbutton in a product page it errors saying 'productpages_stand' does not contain a definition for 'lbnCart' and no extension method 'lbnCart' accepting a first argument of type 'productpages_stand' could be found'.

Am I not able to link a linkbutton to a masterpage from a web form?

The method:

private void updateCartSummary()
    {
        // get number of items in cart and show summary in link button
        ArrayList cart = (ArrayList)Session["CART"];
        int totalItems = cart.Count;
        this.lbnCart.Text = "Cart : " + "(" + totalItems + ")";
    }

Link button from masterpage:

<asp:LinkButton ID="lbnCart" CssClass="shoppingcarttext button small third" runat="server">Cart : (0)</asp:LinkButton>