hello all,

I am writing a trigger that sums up the invoice balances from a managed package object Zuora__ZInvoice__c custom field(Zuora__Balance2__c) to a field open_balance__c on the account. I wrote a corresponnding test class for the same and my test classes assertion method is failing. Am not sure why DML operation is failing.
trigger UpdateOpenBalance on Zuora__ZInvoice__c (after insert, after update) {
  
  Set<Id> Acc = new Set<Id>();
  List<account> ade = new List<account>();
  
   for(Zuora__ZInvoice__c zi : trigger.new){
      if(Trigger.isInsert){
          Acc.add(zi.Zuora__Account__c);
      }
         else if (Trigger.isUpdate){
            Acc.add(zi.Zuora__Account__c);
         }
   }
   
   List<Account> AccountsImpacted = [SELECT Id,Open_Balance__c FROM Account WHERE Id IN :Acc];
   List<Zuora__ZInvoice__c> Invoices = [SELECT Id,Zuora__Balance2__c,Zuora__Account__c FROM Zuora__ZInvoice__c WHERE Zuora__Account__c IN:AccountsImpacted];  
   Set<Zuora__ZInvoice__c>InvToAdd = new Set<Zuora__ZInvoice__c>();
   
   for(Zuora__ZInvoice__c inv : Invoices){
      InvToAdd.add(inv);
   }
   
  if(AccountsImpacted.size() > 0){
   for( Account a : AccountsImpacted){
     a.Open_Balance__c = 0.00;
     for( Zuora__ZInvoice__c zd : InvToAdd){
        a.Open_Balance__c = a.Open_Balance__c + zd.Zuora__Balance2__c ;
        ade.add(a);
     }
   }
  }
   
   update ade ;
}

Apex Test Class
@istest(SeeAllData = true)

 public class TestUpdateOpenBalance{
   public static testmethod void UpdateOpenBalance(){
      Account a = new Account();
     // a.RecordType = 'Client';
      a.name = 'Test Account';
      a.Type_of_Client__c = 'Commercial';
      insert a;
      Profile profileId = [SELECT Id FROM Profile WHERE Name = 'Standard User' LIMIT 1];

      User u = new User();
      u.LastName = 'Test Code';
      u.Email = 'test@test.com';
      u.Alias = 'Tcode';
      u.Username = 'ramupdhay@chatopadhay.com';
      u.CommunityNickname = 'test12';
      u.LocaleSidKey = 'en_US';
      u.TimeZoneSidKey = 'GMT';
      u.ProfileID = profileId.id;
      u.LanguageLocaleKey = 'en_US';
      u.EmailEncodingKey = 'UTF-8';
      insert u;
     
     a.CS_Primary__c = u.id;
     update a;
     
     Zuora__CustomerAccount__c ba = new Zuora__CustomerAccount__c();
     ba.name = 'Test Test';
     ba.Zuora__Account__c = a.id;
     ba.Zuora__Credit_Balance__c = 10.00;
     insert ba;
     
     Zuora__ZInvoice__c zi = new Zuora__ZInvoice__c();
     zi.Name = 'Test Invoice';
     zi.Zuora__Account__c = a.id;
     zi.Zuora__Balance2__c = 10.00;
     insert zi;
     
     zi.Zuora__Balance2__c = 20.00;
     update zi;
     
      System.assertEquals(20,a.open_balance__c);
    
   }
 }