Default gateway

I'm trying to add a default gateway to my BigIP, but perhaps I'm using

the wrong method?

I'm trying something like this (perl):

$soap = SOAP::Lite->uri("urn:iControl:Networking/RouteTable")
      ->proxy("https://$ip/iControl/iControlPortal.cgi") ||
      die "Can't create SOAP object for $ip\n";
$soapResponse = $soap->set_static_route_gateway(
    SOAP::Data->name('routes' => [ { 'destination' => '0.0.0.0',
                                     'netmask'     => '0.0.0.0' } ]),
    SOAP::Data->name('gateways' => [ '1.2.3.1' ]),
);

The response I get is:

SOAP Error: SOAP-ENV:Server; Exception caught in Networking::RouteTable::set_static_route_gateway()
Exception: Common::OperationFailed
primary_error_code   : 16908337 (0x01020031)
secondary_error_code : 0
error_string         : 01020031:3: Opened and closed structures are mixed up.

What am I not reading right? No default route is set (as a matter of fact,

I deleted it with iControl...). Perhaps I should use add_static_route, but

than what should the attributes look like?

Thanks, Michel