Problem with scripting monsters

[The chaser]

Hello everyone:
I'm making a very simple map, "The chasing", with only one monster. The objective is that when the player collides with the script area, a monsters appears. It must be very simple, but i've made the script and it doesn't work.
Here is the script:

void OnStart()
{
AddEntityCollideCallback("Player", "ScriptArea_1", "ActivateMonster", true, 1);

}

void OnEnter ()
{

}

void OnLeave

{

}
void ActivateMonster(string &in asParent, string &in asChild, int alState)
{
SetEntityActive("servant_grunt_2", true);

I don't know what's wrong, so i would like a lot a bit of help.