Lua Creature Spells on Lua

[Siegh]

Im experiencing a very odd bug on lua scripted creature spells and I couldnt find any solution myself and neither with the search function. The problem is the following: whenever a monster cast a damaging lua spell, it sometimes deals the damage correctly, but sometimes it heals the target instead... Ive tried different formulas on the spell but it seems to not affect the damage at all, but only the min/max values on the attack tag on the xml file, and as I said, it doesnt work properly.

Here are the important parts of the scripts:

 local combat = createCombatObject()
setCombatParam(combat, COMBAT_PARAM_TYPE, COMBAT_PHYSICALDAMAGE)
setCombatParam(combat, COMBAT_PARAM_EFFECT, 0)
setCombatFormula(combat, COMBAT_FORMULA_SKILL, -1, -1, -1, -1)

<attack name="blablabla" interval="3750" chance="100" min="-27" max="-100">
</attack>

<instant group="attack" name="blablabla" words="blablabla" lvl="10" mana="0" needtarget="1" range="6" exhaustion="1500" needlearn="1" event="script" value="monsters/blablabla.lua">
</instant>

Im really unsure about what formula I should use, or if I should use a formula at all, so I let the default one I use for player spells. So, please help me fix this nasty bug!