blank page on new register with display field

[jbadiag]

Hi, I have a table where I have a display field with this query in the default value and "eval":

$db =& JFactory::getDBO();
$curs = "{aj_cursosUnio___idCurs_raw}";
$query = "select COUNT(id) from aj_cursosunioinscripcions where aj_cursosunioinscripcions.id_curs = $curs";
$db->setQuery($query);
$result = $db->loadResult();
$inscripcions = $result;
return $inscripcions;

Everything is correct when I see the list and edit the register, but when I try to add a register I get a blank page if this element is enabled. Goes fine if I disable it.
How can I do? Any help?