ClearField function and nil reference

[JonLee-Nest]

Is this usage correct?

pb_instance.ClearField("field_name")

Seems to fail with a nil reference to fields_by_name

        if message_descriptor.fields_by_name[field_name] == nil then
            error('Protocol message has no "' .. field_name .. '" field.')
        end

It looks like the member fields_by_name is never set anywhere, further in the ClearField method there also looks like another potential nil reference as field isn't set either.