I'm having fun trying bulkifying my triggers.  Maybe too much fun!

Anyway, in the code below, about 8 lines form the bottom, there is this line:

        b.Balance__c = b.Balance__c + trans.Qty__c;

This line works fine when adding data through the web interface, but fails with a NullPointerException when I use the data loader.  

If I change the line to:

b.Balance__c =  trans.Qty__c;

Then the data loader does not err out.  So I know the problem is when I try to add in the current balance.  Could it be that my table is named Balance__c and it also also has a column named Balance__c?  But it works through the web interface, just not through the data loader.

Any idea why?  I believe it is thoroughly bulikified.

trigger afterInsertUpdateTrans on Trans__c (after insert, after update) {
    // create a map of existing balance records
    // since locations and equipment are dynamic,
    // balance records are not created until a transaction containing the location and equipment is added
    // there is a single unique balance record for very combination of location and equipment that has a transaction
    // Balance_Key__c is a calculated variable that concatenates the Location Id and Equipment Id
    // Balance_Key__c is defined on both the Trans__c and Balance__c tables
    Map<string,Id> balanceMap = new  Map<string,id>();
    for (Balance__c myBals : [select Balance_Key__c,Id from Balance__c]) {
        balanceMap.put(myBals.Balance_Key__c,myBals.Id);
    }

    string bk;       // to hold the results of the lookup of the key into balanceMap

    // add any missing balance records
    
    List<Balance__c> balanceToInsert = new List<Balance__c>{}; 
    for (Trans__c trans : Trigger.new) 
     {
     bk = balanceMap.get(trans.Balance_Key__c);
     If (bk==null){  // add a row to balanceToInsert
         Balance__c b = new Balance__c();
         b.Location__c = trans.Location__c;
         b.Equipment__c = trans.Equipment__c;
         balanceToInsert.add(b);
     }
    
     }    
    insert balanceToInsert;
    // finished adding any missing balance records



    // make a map of balance records
    map<string, Balance__c> balanceMap2 = new map<string, Balance__c>([select Balance_Key__c, id,Balance__c from Balance__c]);            
    Id balanceObjectId; // to hold the results of the lookup of the key into Balances 

    Map<string,Id> balanceLookup = new  Map<string,id>();
    for (Balance__c myBals : [select Balance_Key__c,Id from Balance__c]) {
        balanceLookup.put(myBals.Balance_Key__c,myBals.Id);
    }

   List<Balance__c> balanceToUpdate = new List<Balance__c>{};

    for (Trans__c trans : Trigger.new) 
 {

     balanceObjectId = balanceLookup.get(trans.Balance_Key__c); 

     Balance__c b = balanceMap2.get(balanceObjectId);
     
     If (trigger.IsInsert){
         // the next line works in the web interface
         // but fails in apex data loader with a System.NullPointerException
         b.Balance__c = b.Balance__c + trans.Qty__c;
         } Else {
         b.Balance__c = b.Balance__c + trans.Qty__c - System.Trigger.oldMap.get(trans.Id).Qty__c;
        }
    balanceToUpdate.add(b);
}
update balanceToUpdate;

}