[SOLVED] Displaying a Custom Button depending on another value

I have a custom button in each row of a data grid but only want to show it if the row isn't > 0

I've added the code to 'before display' in the list page but I either get every row without this code or no buttons when it is.

if ($values['LD_number_of_signatures_required']=0)
{
$pageObject->hideItem("custom_button");
}
else
{
$pageObject->showItem("custom_button");
}