[wierdness with NuSOAP when acting as a SOAP server]

So I'm setting up a web service using NuSOAP to perform a task and return a result. The problem I'm having is that if the elment of the array being returned is a path with "/" in it, the returned result is NULL;

In the listing below, the soap client calling the service listed below with via:
$result = $SOAP_client->call('move_file',$SOAP_params);

Ends up with $result = NULL;

Listing for SOAP server:
[code=php:0]
$server = new soap_server;
$server->register('move_file');

function move_file($from_file, $to_file) {
  blah blah blah

  $result_ary['path/to/file/here/file.ext'] = array ('result'=>'pass','desc'=>'blah blah');
  return $result_ary;
}
remaining code here <snip>
[/code]


if I change the SOAP server so that the first element of the array is something like:

  $result_ary['notApath'] = array ('result'=>'pass','desc'=>'blah blah');

Then I get a result from the soap client as expected ($result contains the array of info).

Is there some reason I can't pass "/"s as part of an array element when using NuSOAP. We used to have this same webservice running as a Perl SOAP::Lite service, (which was able to pass file_paths just fine to the PHP soap client), so I don't think it's something to do with the SOAP specification????