In my Contact trigger I've created a Map based on a query where the Map stores the Account object. It is meant to find the corresponding Account based on the foreign key field customerId, and from which I need to insert the Account's ID and another custom field into the new Contact. When I check on accMap being null, it says it is, but when I use 'isEmpty' it goes into the subsequent code, yet it says there are no values (ie it fails when accessing the values in 'a.Id', saying this statement 'c.AccountId = a.Id;' is referencing a null value).

Can someone tell me what is wrong please?

    Set<string> customerIds = new Set<string>();

    for(Contact c : trigger.new){
        string customerId = c.AccredoCustomerID__c;
        if(!customerIds.contains(customerId)){
            customerIds.add(customerId);
        }
    }

    List<Account> accs = [Select AccredoID__c, Id, AccredoPrimaryContact__c From Account Where AccredoID__c In :customerIds];
    Map<string, Account> accMap = new Map<string, Account>(accs);

    for(Contact c : trigger.new){
        string customerId = c.AccredoCustomerID__c;

       Account a = accMap.get(customerId);

        if(!accMap.isEmpty()){
                Account a = accMap.get(customerId);
                c.AccountId = a.Id;
                if (a.AccredoPrimaryContact__c == c.AccredoId__c){
                    c.Primary_Contact__c = true;
               }
            }
        }
    }