One can divide by a number less than zero. One cannot divide by zero or a null value which is considered zero.
Unless one is dividing by a non-zero constant, one needs to test for a divisor not equal to zero. Since JavaScript is an object orientated language one needs to get the field object and then test the value property of the field.
var nDivisor = this.getField("line_2").value;
event.value = "";
if(nDivisor != 0) event.value = this.getField("line_1").value / nDivisor;