Hey and sorry for the extremely late reply! I've had a loooot going on in my life the last year and a half, so I had to stop developing Mini Myth, but still have hope to return and finish a more complete little RPG system one day!
With that said, to answer your question, DMG should always be minimum "1", even if it is a negative result. This was mostly in order to put an upper limit to DMG on certain weapons, i.e. the staff never being able to deal more than 4 DMG (a limitation due to the use of only a single die). I hope this answers your question :)