The description of the green-flame blade cantrip (SCAG, p. 143) says:
As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell's range, otherwise the spell fails. On a hit, the target suffers the attack's normal effects, and green fire leaps from the target to a different creature of your choice that you can see within 5 feet of it. The second creature takes fire damage equal to your spellcasting ability modifier.
Say you have a negative spellcasting ability modifier; for instance your spellcasting ability is Intelligence, and you have a -1 Intelligence modifier. You cast green-flame blade (and your character is below 5th level). How much damage does the second creature take? None, -1, or round off to 1 damage?