Shadow bug on flipped image

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/fabric.js/2.3.4/fabric.min.js"></script>
<canvas id="c" width="500" height="400" class="c"></canvas>

CSS

canvas {
  border: 1px solid grey;
}

JavaScript

var canvas = document.getElementById('c')
var imageSrc = "http://fabricjs.com/assets/dragon.jpg";
var img = document.createElement('img');
img.onload = function() {
	var ctx = canvas.getContext('2d');
  // comment this line below to solve the problem
  ctx.transform(-1, 0, 0, 1, 400, 0);
  ctx.shadowBlur = 20;
  ctx.shadowOffsetX = 15;
  ctx.shadowOffsetY = 10;
  ctx.shadowColor = 'black';
  ctx.drawImage(img, 10, 10, 380, 210);
}
img.src = imageSrc;