Material-UI. Invalid hook call

I've been following some examples on how to setup Material-UI with React. Whatever I try, I'm getting back the following error:

Error: Minified React error #321; visit https://reactjs.org/docs/error-decoder.html?invariant=321 for the full message or use the non-minified dev environment for full errors and additional helpful warnings.

This seems to be Hook related, but I can't find any reason why this happens.

The component

Button

import React from 'react';
import { withStyles } from '@material-ui/core/styles';
import Button from '@material-ui/core/Button';

const styles = {
    root: {
        background: 'linear-gradient(45deg, #FE6B8B 30%, #FF8E53 90%)',
        border: 0,
        borderRadius: 3,
        boxShadow: '0 3px 5px 2px rgba(255, 105, 135, .3)',
        color: 'white',
        height: 48,
        padding: '0 30px',
    },
};

function ButtonComp(props) {
    const { classes } = props;
    return <Button className={classes.root}>Higher-order component</Button>;
}

export default withStyles(styles)(ButtonComp);

Implemented like this

import * as React from 'react';

import ButtonComp from './ButtonComp';

const App = ({ children }) => {
  return (
    <React.Fragment>
      <ButtonComp />
      <main>
        {children}
      </main>
    </React.Fragment>
  );
};

export default (App);

I'm running on react@16.11.0 and @material-ui/core@4.5.1.

What am I missing here? I can't find the problem at all.