Receiving an error when using getServerSideProps in a Next.js application (TypeError: res.hasHeader is not a function)

I've run into the error below when trying to use getServerSideProps to fetch data from an endpoint.

Error:

ERROR   TypeError: res.hasHeader is not a function
    at sendPayload (/var/task/pages/posts.js:2120:231)
    at renderReqToHTML (/var/task/pages/posts.js:6235:13)
    at process._tickCallback (internal/process/next_tick.js:68:7)

Function:

export const getServerSideProps : GetServerSideProps = async () => {
  const API = 'http://myAPIEndpoint'
  const res = await fetch(API, {
                headers: {
                  'Id': '1234'
                }
              });
 
  const posts = await res.json()
 
  return{
    props: {
      posts
    }
  };
}