Loop through tables on webpage and hide

Hello All,  I am developing a site at the moment in asp.net.  I am trying to dynamically hide all tables that are on a page.  so, I have the page with front end 'asp:table' on (multiple tables on page).

All want to do is 'on page load' hide all the tables on the page.  here is my code and the error i get.  Any help would be very much appreciated :)  I will probably look silly as vb.net is still quite new to me.

Protected Sub Page_Load(sender As Object, e As System.EventArgs) Handles Me.Load

        For Each Table As Table In Page.Controls
            Table.Visible = False
        Next

End sub

-----------------------------------------

Erorr being generated:

System.InvalidCastException was unhandled by user code
  Message=Unable to cast object of type 'ASP.site_master' to type 'System.Web.UI.WebControls.Table'.
  Source=App_Web_dyenvmzk
  StackTrace:
       at Agility.Page_Load(Object sender, EventArgs e) in C:\Documents and Settings\aljacobs\My Documents\Import Ocean Freight Resources\NVOCC\Agility_Logistics.aspx.vb:line 7
       at System.Web.UI.Control.OnLoad(EventArgs e)
       at System.Web.UI.Control.LoadRecursive()
       at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
  InnerException: