Web Service and Groovy Oracle Sales Cloud

Hi all,

i want to update the party usage code , i used this code:

def curPartyId = '300001032631383'
Date curDate = new Date()
  
def partyUsageAssignment =
[     
      PartyId : curPartyId,      
      PartyUsageCode : 'CONTACT',      
      EffectiveStartDate : curDate,    
      CreatedByModule : 'FUSE'
 ]
  
try{
            def ws = adf.webServices.Call_Party_Usage_Service.assignPartyUsage(partyUsageAssignment)
          println('Party Usage Assigned')
          
        }
        catch (Exception e){
            println(e)
            
        }

I have always this error: java.lang.NullPointerException: Cannot invoke method assignPartyUsage() on null object.

Can you help me?
Thanks