When a character with a negative to damage scores a critical hit, the damage is not being calculated correctly. As an example, I have a character in my campaign that has a hawk familiar. It does 1d4-2 damage, which roll 20 is calculating as 2d4-4 damage, which can actually result in a negative number. As is, the game returns -2 to 4 damage. The correct damage should be (1d4-2)(minimum 1) + (1d4-2)(minimum 1). The correct damage spread should be 2 to 4. This only occurs where characters can roll less than a 1 after modifiers on their damage dice.