wrong output when bundle transform `var` to `let`

original code https://github.com/browserify/node-util/blob/master/support/types.js#L25

if (BigIntSupported) {
  var bigIntValue = uncurryThis(BigInt.prototype.valueOf);
}

bundled output transformed code

if (BigIntSupported) {
  bigIntValue = uncurryThis(BigInt.prototype.valueOf);
}
let bigIntValue;

will throw exception when executing code

Uncaught ReferenceError: Cannot access 'bigIntValue' before initialization