I am trying to implement a roll-up summary of the earliest contract start date on the account object using the code outlined here (http://blog.elieperez.net/salesforce-lookup-roll-up-summary/). Below is my trigger.
 

trigger rollupContractsOnAccount on Contract (after insert,after update,after delete,after undelete) {
     
    string mysobjectParent = 'Account',      // Parent sobject API Name
           myrelationName = 'Contract__r', // Api name of the relation between parent and child (ends with __r)
           myformulaParent = 'Customer_Since__c',        // Api name of the number field that will contain the calculation
           mysobjectChild = 'Contract',  // Child sobject API Name
           myparentfield = 'Account', // Api name of the lookup field on chield object
           myfieldChild = 'Subscription_Start_Date__c';          // Api name of the child field to roll up
     
    LookupCalculation.Method method = LookupCalculation.Method.MIN; //Selected method: could be COUNT, SUM, MIN, MAX, AVG
     
    LookupCalculation calculation = new LookupCalculation(mysobjectParent, myrelationName, myformulaParent,
                                                          mysobjectChild, myparentfield, myfieldChild);
    List<sobject> objList = new List<sobject>((List<sobject>) Trigger.new);
    if(Trigger.isDelete)
        objList = Trigger.old;
    if(Trigger.isUpdate)
        objList.addAll((List<sobject>) Trigger.old);
    calculation.calculate(method, objList);
}

This gives me the error:

Error: Invalid Data. 
Review all error messages below to correct your data.
Apex trigger rollupContractsOnAccount caused an unexpected exception, contact your administrator: rollupContractsOnAccount: execution of AfterInsert caused by: System.SObjectException: Invalid field Account for Contract: Class.LookupCalculation.Calculate: line 25, column 1

What am I missing? The error is on "myparentfield," which is correct as Account for the Contract object.