"Failed to load Premain-Class manifest attribute" while trying to get the size of an object using java agent

Question

When i try to run a java program (java -javaagent:size.jar ObjectSizeTest) i get the following error;

Failed to load Premain-Class manifest attribute from D:\workspace\ObjectSizeTest\size.jar
Error occurred during initialization of VM
agent library failed to init: instrument


public class ObjectSizeTest {
    public static void main(String[] args) {
        String s = new String("sai");
        System.out.println(ObjectSizeFetcher.getObjectSize(s));
    }
}

MANIFEST.MF (for size.jar)
Manifest-Version: 1.0
Created-By: 1.5.0_18 (Sun Microsystems Inc.)

Premain-Class: ObjectSizeFetcher

import java.lang.instrument.Instrumentation;

public class ObjectSizeFetcher { private static Instrumentation instrumentation;

public static void premain(String args, Instrumentation inst) {
    instrumentation = inst;
}

public static long getObjectSize(Object o) {
    return instrumentation.getObjectSize(o);
}

}