Test missing [Accountid]:Accountid]

I have a problem with Miising Field. Here is full test: --

private class TestChargeCostUpdate{
static testMethod void testCCUpdate(){
// create data
Account acc = new Account(name='TestAccount');
insert acc;
Opportunity op = new Opportunity(AccountId=acc.id,name='Test Opp',StageName='Provided Quote',CloseDate=Date.toDay());
insert op;
Charge__c es1 = new Charge__c(Opportunity__c = op.id, Monthly_Recurring_Charge__c=1);
insert es1;
Contract contr = new Contract(Opportunity__c = op.id);
insert contr;
}

}

Error:

System.DmlException: Insert failed. First exception on row 0; first error: REQUIRED_FIELD_MISSING, Required fields are missing: [AccountId]: [AccountId]

Thanks for your help.