Short IF notation difference between Java 6 and Java 8

Consider this class, compiled with Maven:

package be.duo.test;

public class Main {

    public static void main(String[] args) {
        Main main = new Main();
        main.execute();
    }

    private void execute() {
        for(int i = 0; i < 10; i++) {
            System.out.println("" + method());
        }
    }

    private int method() {
        return (Math.random() > 0.5d) ? 1 : null;
    }

}

The method() has return type int, which is a primitive type.

Consider the short IF notation in the return statement:

• in Java 6, this will result in a NullPointerException at runtime
• in Java 8, this will result in a compile time error

[ERROR] error: incompatible types: bad type in conditional expression
[ERROR] <null> cannot be converted to int

My source code is thus not forward compatible. Can somebody explain to me why it behaves different?