Filtering walls by Base Constraint: The filter value is not valid for the field and filter type

Question

I'm having difficulty trying to filter out walls by the WALL_BASE_CONSTRAINT. This is the code I currently have. The Build is successful but when I execute the plugin. I come up with the exception: The filter value is not valid for the field and filter type. Parameter name: filter

I have successfully tried this code on other BuiltInParameters like Material_Name, ALL_MODEL_DESCRIPTION.

private static void AddFilterToSchedule(Document doc, ViewSchedule 
schedule)
 {
        string value = "Foundation";

        using (Transaction transaction = new Transaction(schedule.Document, "Filter Material TakeOff"))
        {
            transaction.Start();

            ScheduleDefinition definition = schedule.Definition;
            ScheduleField scheduleField = FindField(schedule, BuiltInParameter.WALL_BASE_CONSTRAINT);

            ScheduleFilter filter = new ScheduleFilter(scheduleField.FieldId, ScheduleFilterType.NotEqual, value);
            definition.AddFilter(filter);

            transaction.Commit();
        }

    }

public static ScheduleField FindField(ViewSchedule schedule, 
BuiltInParameter paramEnum)
    {
        ScheduleDefinition definition = schedule.Definition;
        ScheduleField foundField = null;
        ElementId paramId = new ElementId(paramEnum);

        foreach (ScheduleFieldId fieldId in definition.GetFieldOrder())
        {
            foundField = definition.GetField(fieldId);
            if (foundField.ParameterId == paramId)
            {
                return foundField;
            }
        }
        return foundField;
    }

Answer 1

Where does FindField come from and what does it do?

Have you looked at the ScheduleCreation Revit SDK sample?