Overriding with classes in material-ui v1.0.0-beta-1 shows "the key provided to the classes property is not implemented" warning

I'm trying to override the styling of Material-UI v1 components, using the overriding by classes method.

When I try to override a nested property, for example the :hover pseudo class on the root key I get the following warning:

Warning: Material-UI: the key `.MyButton-root-w:hover` provided to
the classes property object is not implemented in Button.
You can only overrides one of the following: 

See for example:

import React from "react";
import { createStyleSheet, withStyles } from "material-ui/styles";
import Button from "material-ui/Button";

const buttonStyle = createStyleSheet("MyButton", {
  root: {
    backgroundColor: "#f99",
    "&:hover": {
      backgroundColor: "#99f"
    }
  }
});

export default withStyles(buttonStyle)(Button);

Or see it in action at https://codesandbox.io/s/gRgGrYvr

As the example button does get a different background color when hovering, I was wondering if this is an issue in Material-UI or if I don't yet fully grasp how to override its styling.