Using the FrequencyTimer2 library

Hi! I'm running arduino 0008 (SVN checkout) from the commandline under linux, and am using an Arduino with ATmega168.

Now I'm trying to use the FrequencyTimer2 library (though translated to C by me) to get a timer interrupt. This works fine, but I'm a bit confused about the timing, see below...

I use setPeriod(1000000L/100); to get an interrupt at 100Hz, which should be correct since setPeriod takes the argument in uS, right?
Then in my handler I do like this:

void tmr_handler(void) {
    static int counter = 0;
    if(++counter==100) counter=0;
    if(counter==0)
        digitalWrite(13,HIGH);
    if(counter==50)
        digitalWrite(13,LOW);
}

I thought this should light up my LED on pin13 once per second, but instead it lights up twice a second.
Am I dumb? Or is there some register somewhere that needs tweaking?

/Jonatan