When checking a condition with a conditional format, it breaks the grid if it equals NULL?

All i get is LOADING…

$f = array();
$f[“column”] = “checked_by”;
$f[“op”] = “=”;
$f[“value”] = “EST”;
$f[“css”] = “‘background-color’:’#9900FF’, ‘color’:’black'”;
$f_conditions[] = $f;

If I REM the above out, or the checked_by does not contain NULL it works fine. Any idea how to fixe this?

Thanks,

Tim