Why does flexbox stretch my image rather than retaining aspect ratio?

Flexbox has this behaviour where it stretches images to their natural height. In other words, if I have a flexbox container with a child image, and I resize the width of that image, the height doesn’t resize at all and the image gets stretched.

div {
  display: flex; 
  flex-wrap: wrap;
}

img {
  width: 50%
}

<div>
    <img src="https://i.imgur.com/KAthy7g.jpg" >
    <h1>Heading</h1>
    <p>Paragraph</p>
</div>

What causes this?