What's Legal About Legal Moralism?

If legal moralism posits a normative connection between culpable wrongdoing and punishment, what should legal moralists say about cases in which responsible agents commit culpable wrongs that have not been proscribed ex ante by the state in which they occur? More succinctly, what is the status of the principle of legality according to legal moralists? I argue that the absence of law typically (but perhaps not always) provides a sufficient non-desert basis to withhold punishment from culpable wrongdoers whose punishment is deserved. I critically examine the probable implications of this way of accounting for the significance of legality.