Octal numbers in babel

I'm playing with babel-cli. I installed the ES2015 extension and it works well. For example, the following snippet:

let square = x => x * x;

...is converted into:

"use strict";

var square = function square(x) {
  return x * x;
};

However I have trouble when using octal numbers. For example:

let mode = 0777;

throws me an error:

SyntaxError: index.js: Invalid number (1:11)
> 1 | let mode = 0777;
    |            ^

  2 |

It looks like it doesn't like the numbers starting with 0 (octal numbers). How can I solve this?

In fact, such numbers appear not in my code but in one of the dependencies.

Is it a babel bug or a feature? What is the workaround/solution?