You'll notice that the character uses the "not enough mana" sound file, and this is mirrored in the combat log (as stated above). However, it is based on your rune cooldowns. If your blood or unholy runes are on cooldown (and, of course, when both are on cooldown), Necrosis will continue to fail due to not enough mana, despite the DK class not using mana at all and the skill not requiring any runes.
If you run about and don't use your runes, Necrosis will be applied on hits, and it won't use any runes. Mind Shock is similar in that, while it uses runic power, it will complain when Frost runes are on cooldown despite not needing them.