protobuf: Difference between new Prototype() and Prototype.Clear()

for two pieces of codes

for (loop) {
    ProtoType instance;
    // do something with instance;
}

/* --------------- */

ProtoType instance;
for (loop) {
    // do something with instance;
    instance.Clear();
}

I got core dump with the latter one. So what's the difference between these two pieces of codes? What does Prototype.Clear() really do?