The Controls collection cannot be modified because the control contains code blocks (i.e. <% ... %>).

hi all

may i know what is this problem "The Controls collection cannot be modified because the control contains code blocks (i.e. <% ... %>). "

this problem occur where i call this javascript function :
function fnUpdate(iHeight, iWidth,Side)
{
if(Side=='front')
{
var editor1 = document.getElementById('<% = ceFrontDesign.ClientID %>');
}

else

{

alert(Side);

}

...