When using a canvas component, mountedImages[i] is undefined

Use case like so:

``` js
import { Button } from 'reapp-kit';
import { Surface, Text } from 'react-canvas';

export default class extends React.Component {
render() {
return (


Here is some text


hello world


);
}
};

const styles = {
width: window.innerWidth,
height: window.innerHeight,

text: {
top: 20,
left: 20,
width: window.innerWidth,
height: 20,
lineHeight: 20,
fontSize: 12
}
}
```

Button is just a simple surface, but its defined in an different module. That module does a require('react-canvas') as well though.

Error is on this line:
https://github.com/Flipboard/react-canvas/blob/3ac33e05ff9364a570613e3d57d1877563163a23/lib/ContainerMixin.js#L121

When inspecting that error, mountedImages looks like:

``` js
[RenderLayer, ""]
```

Seems like the problem is that it's expecting a RenderLayer but it's getting a string (the canvas string above).

Any ideas?