2A motor shield shorts when setting motors to reverse

I am having some problems with the motorshield. if I set the direction of both motors to high, everything is fine, when i switch them to low, to make the motors go backwards, there is a short and the arduino shuts off. Am I doing something wrong?
Code i have used:
[code]int el = 6;
int ml = 7;
int er = 5;
int mr = 4;
int btpin = 12;
char val;

void setup(){
pinMode(ml, OUTPUT);
pinMode(mr, OUTPUT);
pinMode(btpin, OUTPUT);
digitalWrite(btpin, HIGH);
Serial.begin(9600);
}

void loop(){
if(Serial.available()){;}
val = Serial.read();
if (val == 'w'){
digitalWrite(ml,HIGH);
digitalWrite(mr,HIGH);
analogWrite(el, 255);
analogWrite(er, 255);
}
if (val == 's'){
digitalWrite(ml,LOW);
digitalWrite(mr,LOW);
analogWrite(el, 255);
analogWrite(er, 255);
}
}
[/code]
I am using the motorshield in PWM mode and the only thing i have wired up (besides motors) is a bluetooth module.