Accessing External List field properties with CSOM

Hi,

I have an external list with a DateTime field on. I want to access the field properties such as DisplayFormat, DefaultValue etc on the field. I understand that some of these will be null as they come from an external source but I cannot even access the properties by loading them from the context:

FieldDateTime dateTime = myExtlist.Fields[4] as FieldDateTime;
            if (dateTime != null)
            {
                ctx.Load(dateTime, dt => dt.DateTimeCalendarType, dt=> dt.DisplayFormat, dt => dt.FriendlyDisplayFormat);
                ctx.ExecuteQuery();
            }

I am getting the exception: Invalid field name. {00000000-0000-0000-0000-000000000000}  /sites/FieldTest/Lists/myExtList

Using a different approach, I can access the more generic field properties by doing this:

ctx.Load(myExtList,l => l.Title, l => l.Fields.Include(
                    f => f.Id,
                    f => f.Title,
                    f => f.TypeAsString,
                    f => f.DefaultValue,
                    f => f.Description,
                    f => f.EnforceUniqueValues,
                    f => f.FieldTypeKind,
                    f => f.FromBaseType,
                    f => f.Group,
                    f => f.Hidden,
                    f => f.ReadOnlyField,
                    f => f.Required,
                    f => f.Scope,
                    f => f.Sealed,
                    f => f.InternalName,
                    f => f.StaticName,
                    f => f.SchemaXml));

My propblem is, I need to get the field-specific properties. DateTime is one example but, for instance, Number fields will have min and max values etc which I won't reach either using the generic approach. 

Has anyone done this before?

Thanks

Mike