src/main/java/Rational/Rational.java

@@ -86,7 +86,7 @@ public class Rational {
     // COMPLETED - Instance method - this / other
     public Rational divide(Rational other) {
         if (other.numerator == 0) {
-            return null;
+            throw new ArithmeticException("Invalid division. Denominator will be zero");
         }
 
         int newNumerator = this.numerator * other.denominator;