I'm trying to find all accounts which have zimbraNotes field equals "" (blank) using the following filter, but it's not working:

Code:
SearchOptions arg=new SearchOptions();

arg.setQuery("zimbraNotes='' ");
String attRet[]=null;
arg.setReturnAttrs(attRet);
arg.setFlags(Provisioning.SA_ACCOUNT_FLAG);

List<NamedEntry> list = sp.searchDirectory(arg);
How to do that?