gistfile1.cpp

bool Unit::IsSpellCrit(...)
{
....
    // Creatures cant crit with spells
    if (GetTypeId() == TYPEID_UNIT && (((Creature*)this)->GetSubtype() == CREATURE_SUBTYPE_GENERIC // If its normal creature
        || (((Creature*)this)->GetSubtype() == CREATURE_SUBTYPE_TEMPORARY_SUMMON && !((Creature*)this)->GetOwnerGUID()))  // or temporary summon without owner, hope this is correct
        && spellProto->DmgClass == SPELL_DAMAGE_CLASS_MAGIC) // Also affect only magic
        return false;
....
}