Life Drain/Siphon Mana "Must" target enemy

I'm trying to create a spell that basically does the same thing as siphon mana (supposedly) does for allies, but with life instead of mana. I was of the assumption that merely changing the values on siphon mana so that the mana-related values were zero and the "life transferred" values were positive would do this. However, no matter what targets I set the spell as able to hit, in-game it just tells me "must target enemy units". In fact, even the completely unedited, default "siphon mana" for the blood mage seems to do this.

I'm all out of ideas, so I was wondering if this is a glitch I can somehow get rid of, or if it's something I can't change and I'll need to find some kind of workaround using a trigger or something. If the latter, I have a vague idea of how to do it but might need help finding the "channel" ability so I can actually edit or copy it and use it in a trigger.

You must set the correct value under gameplay constants to allow siphon mana/life type spells to target allies.

Um...right. Because I had just joined the forum and didn't realize I had to be recognized by an admin as a non-spambot before posting, this is kind of really outdated...sorry. I got some help on the IRC and it's fixed now.

Thank you anyway.