How to handle empty value in nlobjSearchFilter

A basic question: when I search something in inventory numbers record, I defined filter like this:
     var filter = new nlobjSearchFilter('item', null, 'is', '3966');
It run good. 'item' is the internal id of this record.

But there are following situations:
1) When I change value to nothing, like this:
    var filter = new nlobjSearchFilter('item', null, 'is', '');
there's a '[LEFT]SSS_INVALID_SRCH_OPERATOR' [/LEFT]error message.

2) When I change value to something with letters, like this:
    var filter = new nlobjSearchFilter('item', null, 'is', 'abc');
there's a 1000 results.

Why does that happen? How to handle empty value. Currently I use an impossible number like '-99999999' to avoid error.