SoapFault and encoding option

I've got some strange behavior when using SoapFault object with non default 
encoding option:

$server=new SoapServer("test3.wsdl",array('encoding'=>'ISO-8859-2'));

When function a() is defined as follows:

function a() {
 return  " ... some iso-8859-2 characters ...";
}

everything works as expected

when I've changed it to:

function a() {
 throw new SoapFault("Server"," ... some iso-8859-2 characters ...");
}


I've got:

Fatal error: Uncaught SoapFault exception: [Sender] looks like we got no XML 
document

with plain ASCII everything is fine.

JG