Property value expected type of string but got null

• babel-plugin-codegen version: ^3.0.0
• node version: 10.12.0
• npm (or yarn) version: 6.4.1

Relevant code or config

// @codegen

const hello = 'hello';

module.exports = `
  export default {};
`;

I was expecting the simple code above to work but it just gives the error mentioned in the title.

However, if we remove the line const hello = 'hello', everything works as expected and export default {}; it's correctly generated