Assassin's Mark does not grant power charge with DoT

The text on the gem says that "Killing the cursed enemy will grant life and mana, and a power charge." The text further says that "Cursed enemies have a 100% chance to grant a power charge when slain."

However, when a marked enemy dies to damage over time (I've only tested with poison) they do not grant a charge at all. This feels unexpected as the source of the poison (in this case) is myself, not a totem or minion, and the poison was even applied by a melee attack (Viper Strike). Nothing on the gem (or the wiki) indicates that the monster must be slain *by a hit* in order to receive the charge (I haven't checked the life/mana gain on kill, but reasonably expect that if I've not getting the charge, I'm not getting those, either).