Deprecated typography warning when using custom theme in Material-UI

In the following code code, I am using a custom theme in Material-UI:

import React from "react";
import ReactDOM from "react-dom";
import { MuiThemeProvider, createMuiTheme } from "@material-ui/core/styles";
import Button from "@material-ui/core/Button";
import { purple, green } from "@material-ui/core/colors";

const theme = createMuiTheme({
  palette: {
    primary: purple,
    secondary: green
  }
});

function App() {
  return (
    <MuiThemeProvider theme={theme}>
      <Button color="primary">Button1</Button>
      <Button color="secondary">Button2</Button>
    </MuiThemeProvider>
  );
}

ReactDOM.render(<App />, document.getElementById("root"));

I am getting the following warning: Warning: Material-UI: you are using the deprecated typography variants that will be removed in the next major release.

I am not using any typography (let alone any deprecated typography variant)

Why am I getting this warning? Can someone please explain.
Full demo at codesandbox: https://codesandbox.io/s/r5v9pjxnq4

Minimal repro code: https://codesandbox.io/s/p9628o7wlj
(Turns out just using createMuiTheme also gives the warning!)