Babel error: repl: We found a path that isn't a NodePath instance. Possiblly due to bad serialisation

When running the code snippet (taken from a MDN page regarding the let keyword in ES2015) through the online Babel transpiler, it fails with the following error:

repl: We found a path that isn't a NodePath instance. Possiblly due to bad serialisation.

Code

var list = document.getElementById("list");

for (var i = 1; i <= 5; i++) {
  var item = document.createElement("LI");
  item.appendChild(document.createTextNode("Item " + i));

  let j = i;
  item.onclick = function (ev) {
    console.log("Item " + j + " is clicked.");
  };
  list.appendChild(item);
}

Why is the error thrown and what can I do about?