Calculating fields with if/else

Help!  I am such a newbie and I have tried to utilize the other posts which were similar.

Here is the simple script.

var C5 = this.getField("C5").value

if(C5=0) {

event.value=0;

}else {

event.value=(1000/C5);

}

When I change the value in C5, nothing changes in the calculated field it just keep displaying 'Infinity'.

I am using acrobat for the form creation.