api - SOAP PHP request error

I'm using a 3rd party api to request balances. I'm getting numerous errors on different tries but I know I'm close. here's my code for the call...

<?php 
try {   
$wsdl_url = 'https://www.domain.com/SOAP?wsdl';
$UserID = 'UserID';
$Pwd= 'Password';
$client = new SOAPClient($wsdl_url);
$headerbody = array( 
                     'UsernameToken'=>array('Username'=>$UserID, 
                                             'Password'=>$Pwd)); 

//Create Soap Header.        
$header = new SOAPHeader('wsse', 'Security', $headerbody);        

//set the Headers of Soap Client. 
$client->__setSoapHeaders($header); 
//$client->__getLastResponse();
$BalanceInquiryRequest->AccountNo="7777700020";
$BalanceInquiryRequest->RetailerId="0123";

$getBalance = $client->balanceInquiry($BalanceInquiryRequest);
print_r($getBalance); 
} catch (SoapFault $fault) {
echo $fault->faultcode . "-" . $fault->faultstring;  
//echo "REQUEST:\n" . htmlentities($client->__getLastRequest()) . "\n";
}

?>
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the output of this is...

env:Server-java.lang.NullPointerException

and here's the xml for the request...

<soapenv:Envelope xmlns:req="http://domain.com" xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/">
   <soapenv:Header>
      <wsse:Security soapenv:mustUnderstand="0" xmlns:wsse="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-secext-1.0.xsd" xmlns:wsu="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-wssecurity-utility-1.0.xsd">
         <wsse:UsernameToken wsu:Id="UsernameToken-1">
            <wsse:Username>Username</wsse:Username>
            <wsse:Password Type="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-username-token-profile-1.0#PasswordText">Password</wsse:Password>
            <wsse:Nonce EncodingType="http://docs.oasis-open.org/wss/2004/01/oasis-200401-wss-soap-message-security-1.0#Base64Binary">5Ht+O9hKvAEXhNJZSTLg==</wsse:Nonce>
            <wsu:Created>2012-03-27T15:31:01.792Z</wsu:Created>
         </wsse:UsernameToken>
      </wsse:Security>
   </soapenv:Header>
   <soapenv:Body>
      <req:BalanceInquiryRequest>
         <req:AccountNo>7777700020</req:AccountNo>
         <req:RetailerId>0123</req:RetailerId>
      </req:BalanceInquiryRequest>
   </soapenv:Body>
</soapenv:Envelope>
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