Update Custom Field in Subscription from Salesforce via Apex callout

[KHarrington]

I'm trying to udpate one custom field via an apex call from Salesforce so that users don't have to go through the whole quote process to update a single custom text field.  I am using the following code:

global class Z_UpdateSubCustomFieldsInvokable {
        @InvocableMethod
        global static void Z_UpdateSubscriptionCustomFieldsInvokable(List<Id> subscriptionIds){
            System.debug(SubscriptionIds);
            updateZuoraSubscription(subscriptionIds);
        }
        @future(callout=true)
        public static void updateZuoraSubscription(List<Id> subscriptionIds){
            List<Zuora__Subscription__c> Subscription = [SELECT Zuora__External_Id__c, PONumber__c FROM Zuora__Subscription__c where Zuora__External_Id__c IN :subscriptionIds];
            for (Zuora__Subscription__c sub : subscription){
                makeCallout(sub.Zuora__External_Id__c, sub.PONumber__c);
            }
        }
    
        public static void makeCallout(Id SubscriptionId, String PONumber){
            Zuora.ZApi zApiInstance = new Zuora.ZApi();
            zApiInstance.zlogin();
            
            Zuora.zObject subscription = new Zuora.zObject('Subscription');
            subscription.setValue('id', SubscriptionId);
            subscription.setValue('PONumber__c__c', PONumber);
            List<Zuora.zObject> objs = new List<Zuora.zObject> {subscription};
            
            List<Zuora.zApi.SaveResult> results = zApiInstance.zupdate(objs);
        }
    }

but I get the following error upon execution. I did confirm the id in the error matches the zuora id for the subscription, is the error somewhere in my syntax?

16:25:46:028 FATAL_ERROR System.StringException: Invalid id: 2c92c0f8739445ac017397a8d30873fa