Need help with INVALID_CROSS_REFERENCE_KEY, invalid cross reference id error message please

Hi Everyone,

I've written the below code but when the 'campainsToUpdate' list (Line 43) has more than one record in it I get this error:

System.DmlException: Update failed. First exception on row 0 with id 70120000000dolQAAQ; first error: INVALID_CROSS_REFERENCE_KEY, invalid cross reference id: []

If I limit the list to one record it will work and update the record any ideas what I'm doing wrong?

Public Class campaignIterationCounter{
        Public Static Void iterationCounter(list<campaign> campaignsToUpdate){
            string prevEventSeries;
            integer i = 0;
            List<Campaign> updateCampaigns = new list<campaign>();
            for(campaign c: campaignsToUpdate){
                if(prevEventSeries == c.Event_Series__c){
                    system.debug('Prev Event Series = to campaign event series');
                    i = i++;
                    c.How_Many_Times_Has_Event_Run__c = i;                   
                    updateCampaigns.add(c);                                       
                }
                Else if(string.isblank(PrevEventSeries) || PrevEventSeries != C.Event_Series__c){
                    system.debug('Prev ES not = to Campaign ES');
                    i = 1;
                    c.How_Many_Times_Has_Event_Run__c = i;
                    prevEventSeries = c.Event_Series__c;
                    updateCampaigns.add(c);
                }          
            }
            system.debug(UpdateCampaigns);
            update updateCampaigns;
        }
        Public Static Void listDefinition(list<campaign> newCampaigns){
            system.debug(newCampaigns);
            map<String, campaign> uniqueEventSeries = new map<string, campaign>();
            for(campaign c: newCampaigns){
                if(!uniqueEventSeries.containsKey(c.event_series__c)) {                
                    uniqueEventSeries.put(c.event_series__c, c);
                }                
            }
            list<campaign> campaignsToUpdate = [SELECT ID, Event_Series__c from Campaign where Event_Series__C in :uniqueEventSeries.keyset() Order by Event_Series__c, CreatedDate ASC];
            system.debug(campaignsToUpdate);
            System.debug('Passing List to Iteration Counter');
            iterationCounter(CampaignsToUpdate);
        }
}