Writing a script in a PDF form so colour of text changes based on a value.

I have a simple script but not sure what is wrong. Basically if the number input to the form is less than, for example, 5. Then turn the font colour red. It works but.. not if you input 0.

var A = Number(this.getField("Quantity").valueAsString);

if(A){
 if(A<5)
 event.target.textColor = color.red;
 else
 event.target.textColor = color.black;}