Karthus E not stacking Tear of the Goddess on initial cast.

Karthus toggle from off to on, costs mana, however, this does not seem to stack with tear of the goddess. Tear defines a stack as any ability or"mana expendsure". Not sure if this same interaction is true for singed. Might be just the first cost doesnt stack while all others after the first do, i am not sure. Please check this out, thank you.