[Konstantinos]

It's caused because of active_character[playerid].

pawn Код:

new slot = GetFactionSlot(CharacterInfo[playerid][active_character[playerid]][cFaction]);

Let's say, you declared it as:

pawn Код:

new CharacterInfo[MAX_PLAYERS][25][some_enum_here];

You need to check if active_character[playerid] is between 0-24. Change accordingly to the value of the dimension.

pawn Код:

public IsLaw(playerid)
{
    if (0 <= active_character[playerid] < 25)
    {
        new slot = GetFactionSlot(CharacterInfo[playerid][active_character[playerid]][cFaction]);
        if(slot > -1 && FactionInfo[slot][fType] == FAC_TYPE_LAW) return true;
    }
    return false;
}

If IsLaw is not called by a timer or Call Remote/Local Function, then just use stock instead of forward/public.