Spring Assert.notNull--IllegalArgumentException

Exception in thread "main" java.lang.IllegalArgumentException: Source must not be null
    at org.springframework.util.Assert.notNull(Assert.java:115)
    at Test.getaVoid(Test.java:11)
    at Test.main(Test.java:6)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:147)

import org.springframework.util.Assert;

public class Test {

    public static void main(String[] args) {
        getaVoid();
        System.out.println("hello");
    }

    private static void getaVoid() {
        Assert.notNull(null, "Source must not be null");
    }

}