Hiya folks, can someone help me figure out what the heck is wrong? My code passes all my unit tests, and eveyrthing works as it should in the org...

The error:
Challenge Not yet complete... here's what's wrong: 
There was an unexpected error in your org which is preventing this assessment check from completing: System.DmlException: Update failed. First exception on row 0 with id 500360000034o9zAAA; first error: CANNOT_INSERT_UPDATE_ACTIVATE_ENTITY, MaintenanceRequest: execution of AfterUpdate caused by: System.NullPointerException: Argument cannot be null. Class.MaintenanceRequestHelper.updateWorkOrders: line 33, column 1 Trigger.MaintenanceRequest: line 12, column 1: []

The code:

public class MaintenanceRequestHelper {
    
    public static void updateWorkOrders(List<Case> maintenanceRequests){

        Set<Case> targetRequests = new Set<Case>();
        for (Case mr : maintenanceRequests)
            if (('Repair' == mr.Type || 'Routine Maintenance' == mr.Type) && 'Closed' == mr.Status)
                targetRequests.add(mr);

        List<AggregateResult> results = [
            SELECT Maintenance_Request__c, MIN(Equipment__r.Maintenance_Cycle__c)
            FROM Work_Part__c
            WHERE Maintenance_Request__c IN :targetRequests
            GROUP BY Maintenance_Request__c
        ];

        Map<Id, Integer> requestToMinCycle = new Map<Id, Integer>();
        for (AggregateResult ar : results)
            requestToMinCycle.put(
                (Id)ar.get('Maintenance_Request__c'),
                ((Double)ar.get('expr0')).intValue()
            );

        List<Case> workOrders = new List<Case>();
        for (Case t : targetRequests)
            if (('Repair' == t.Type || 'Routine Maintenance' == t.Type) && 'Closed' == t.Status)
                workOrders.add(new Case(
                    Vehicle__c=t.Vehicle__c,
                    Type='Routine Maintenance',
                    Subject=t.Subject,
                    Status='New',
                    Date_Reported__c=Date.today(),
                    Date_Due__c=Date.today().addDays(requestToMinCycle.get(t.Id))
                ));
        
        if (!workOrders.isEmpty())
            insert workOrders;
    } 
}

The error appears to be targeting my line where assign the new Date_Due__c, implying the Case is not in my requestToMinCycle map. But I mean... everything works! And the value is populated correctly in both my unit and practical tests! So what the heck?!

Any thoughts?

Thanks very much!