Hi all,

I written one trigger and test class getting this error. 
System.TypeException: Invalid id value for this SObject type

Trigger:-
----------
trigger EscalateCase on Escalation__c (after update) {
IF(Trigger.size > 1){return;}
IF(Trigger.new[0].Reason_For_Escalation__c == null || Trigger.new[0].Escalation_Comments__c == null || Trigger.new[0].Escalate_To_Management__c == false || Trigger.new[0].Case__c == null){return;}
Case c = new case(ID = trigger.new[0].Case__c, Escalate_To_Management__c = trigger.new[0].Escalate_To_Management__c, Reason_For_Escalation__c = trigger.new[0].Reason_For_Escalation__c, Escalation_Comments__c = trigger.new[0].Escalation_Comments__c);
update c;
}

Test class:-
--------------

@isTest
private class EscalateCaseTestClass{
@isTest static void Esc(){

Account a = new Account(Name='ContractBookingPeriodProcessingTest',Geography__c='TestGeography',
        Territory_Id__c='TestTerritory',
            Territory_Overlay__c='TestTerritoryOverlay',Renewals_Team__c='TestRenewalsTeam',
            Renewals_Account_Manager__c='TestRenewalsAccountManager');
        insert a;
        Contract c = new Contract(AccountId=a.id,Number_of_Booking_Periods_Records__c = 0, Number_of_Booking_Periods__c = 1,
                                    Status='Create', Finance_Charges__c ='0.5%',
                                    Contract_Term_Start_Date__c=date.valueOf('2013-01-01'),
                                    Contract_Term_End_Date__c=date.valueOf('2099-12-31')                    
                                    );
        insert c;

Case case1 = new case(Id=a.id,Escalate_To_Management__c =true,Reason_For_Escalation__c = 'TestReason' ,Escalation_Comments__c = 'TestEscalation' );
insert case1;

case1.Reason_For_Escalation__c = 'TestReasonForEscalation';
case1.Escalation_Comments__c = 'TestEscalationComments';
update case1;
}
}

Thanks
kumar