debugging help wanted

Hello,

I've got quite strange results whith the following routine:

foreach ($result as $key => $val) {
echo "$key => $val<br>";
$smarty->assign($key . '_value',$val);
$smarty->assign($key . '_selected',$val);
}

One of the keys is 'ac_type' and $val is 'xxx'.
With Smarty-debugging on, I get :

'ac_type_value' empty
'ac_type_selected' 'xxx'

All other assignments work as wanted...

Is there a smart way to find out, whether there has been a conflicting 
assignment to 'ac_type_value' and at what point of script processing?

Peggy