Problem with changing last dunned date

I am trying to change / remove the last dunned date and existing dunning level so that we can re-set the dunning process.

This works if taking the dunning level back from 2 to 1, and entering a "dummy" date for last dunned but not if we are taking it back from 1 to 0, with no last dunned date. In the latter case, it is still producing a level 2 output.

Can anyone suggest how to re-set the dunning to level 0 (not yet dunned)?