Fetch TypeError: Failed to execute 'fetch' on 'Window': illegal call

I am trying to use fetch to make calls to the backend from react without libs like Axios.

api.ts

export const sendSuggestion = ((data: any): Promise<any> => {
console.log(JSON.stringify(data));
const apiUrl = `/api/suggest/`;
return fetch(apiUrl, {
    method: 'POST',
    headers: {'Content-Type': 'application/json'},
    body: JSON.stringify(data)
}).then(checkStatus)
    .then(r => r.json())
    .then(data => {
        console.log(data);
    });
});

const checkStatus = ((response: any) => {
  if (response.ok) {
    return response;
  } else {
    var error = new Error(response.statusText);
    console.log(error);
  //return Promise.reject(error);
  }
})

      

        
        
        
      

    

Also I include an npm module which polyfill https://www.npmjs.com/package/unfetch and imports it into my code

 import fetch from 'unfetch'


 console.log(fetch) returns in console:
 function fetch() { [native code] } 

      

        
        
        
      

    

I can't figure out what the problem is.