This warning was fixed for me by adding an .babelrc file in the project main folder, with the following content:
{
"presets": ["next/babel"],
"plugins": [["styled-components", { "ssr": true }]]
}
See following link for an example:
https://github.com/nblthree/nextjs-with-material-ui-and-styled-components/blob/master/.babelrc