I've got a trigger on contact to update an account field with the count of child contacts meeting a certain criteria.  My trigger works upon testing, but my test class is giving me some huge errors.  FATAL_ERROR System.DmlException: Insert failed. First exception on row 0; first error: CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY, BuyingCenterCount: execution of AfterInsert

Here is my Trigger:

trigger BuyingCenterCount on Contact (after insert, after update, after delete, after undelete) {
   
   
 
    List<Contact> contacts = Trigger.isDelete ? Trigger.old : Trigger.new;

  
    Set<Id> acctIds = new Set<Id>();
   
    for (Contact c : contacts) {
     
        if (c.AccountId != null) {
            acctIds.add(c.AccountId);
        }
    }
   
    List<Account> acctsToRollup = new List<Account>();
    
    
    for (AggregateResult ar : [SELECT AccountId AcctId, Count(id) ContactCount 
                               FROM Contact 
                               WHERE AccountId in: acctIds 
                               AND OP_Buying_Center__c = True
                               GROUP BY AccountId]){
        Account a = new Account();
        a.Id = (Id) ar.get('AcctId'); 
        a.Number_of_OP_BC_Contacts__c = (Integer) ar.get('ContactCount');
        acctsToRollup.add(a);
    }
    
    
    update acctsToRollup;

}





Here is my test class - I've simplified it to only try and insert an account and contact for now but still getting the same error. 

@isTest
private class BuyingCenterCountTest {
    
    @isTest static void BuyingCenterCount() {
        
        Account t = New Account();
        t.Name = 'Tester';
        t.BillingCountry = 'UK';
        t.Website = 'www.creep.gov';
        
        insert t;
        
        Contact c = New Contact();
        c.LastName = 'Terry';
        c.OP_Buying_Center__c = True;
        c.AccountId = t.Id;
      
        insert c;
    }

}



Does anyone have experience with these errors?