Upserting a set of Objects

/*
* This code aims to test the Test Database.Upsert(SObject[] soList, Schema.SObjectField field, Boolean all_or_none) 
* method. Described by this reference: http://www.salesforce.com/us/developer/docs/dbcom_apex250/Content/apex_dml_upsert.htm
* 
* For the moment, the test fails, by retreiving the following error: "Upsert requires a concrete SObject type"
* ***Considerations:
* Please substitute the id values in the set accIds, with account IDs from your org.
*/

// 1. Retrieve Accounts
Set<String> accIds = New set<String>{'001w000001JklMQAAZ', '001w000001DdXjG'};
List<Account> al = [Select id, Name, Test_External_Id__c From Account where id in: accIds];
System.debug('1. Al: ' + al);

// 2. Modifying values:
al[0].Name = 'Enver Hoxha';
al[1].Name = 'Nexhmije Hoxha';
System.debug('2. Al Updated: ' + al);

// 3. TypeCasting in Sobjects
List<SObject> soL = new List<SObject>();
for(Account a: al){
	sol.add((SObject)a);
}

// 4. Tentando upsert
Schema.SObjectField field = Account.Fields.Test_External_Id__c;
Database.UpsertResult [] ups = Database.upsert(sol, field, false);// ***Code fails HERE
System.debug('4. Esito Upsert: ' + ups);