VM: weird behaviour in runInContext/runInThisContext

While working on my electron based REPL application, I noticed a weird behaviour in vm.runInContext.

Program

Desktop 🙈 ₹ cat repl-vm-strict.js
'use strict';
const vm = require('vm');
const ctx = vm.createContext();

try {
  const result = vm.runInContext('"use strict"; x = 1', ctx);
} catch(e) {
  console.log(e.stack);
  console.log('x is', ctx.x)
}

Output

Desktop 🙈 ₹ node repl-vm-strict.js
ReferenceError: x is not defined
    at evalmachine.<anonymous>:1:17
    at Object.exports.runInContext (vm.js:44:17)
    at Object.<anonymous> (/Users/princejohnwesley/Desktop/repl-vm-strict.js:7:21)
    at Module._compile (module.js:413:34)
    at Object.Module._extensions..js (module.js:422:10)
    at Module.load (module.js:357:32)
    at Function.Module._load (module.js:314:12)
    at Function.Module.runMain (module.js:447:10)
    at startup (node.js:140:18)
    at node.js:1001:3
x is 1

Behaviour

vm.runInThisContext/vm.runInContext throws error. ✅
context is also updated as if variable is created with var or let binding ❌

Versions

Tested versions: v5.6.0 & v5.1.1

Platform

Desktop 🙈 ₹ uname -a
Darwin Princes-MacBook-Pro.local 15.3.0 Darwin Kernel Version 15.3.0: Thu Dec 10 18:40:58 PST 2015; root:xnu-3248.30.4~1/RELEASE_X86_64 x86_64

Subsystem

vm

repl with strict is affected by this issue.