Node.js oAuth - unsupported_grant_type on Step 4


I am trying to implement the oAuth2 flow using Node.js to enable my users to connect their Patreon account to my site to prove they are indeed paying. I am following the oAuth implementation guide but at step 4 the only response I get from the API is

{ error: 'unsupported_grant_type' }

My code is

const response = await fetch("", {
    method: "POST",
    headers: { "Content-Type": "application/x-www-form-encoded" },
      "code=" +
      code +
      "&grant_type=authorization_code&client_id=" +
      PA_APP_ID +
      "&client_secret=" +
      "&redirect_url=" +

All the variables are set correctly and according to the docs the required grant type is indeed “authorization_code”. So … what am I doing wrong? I have done several other oAuth implementations and this is the only time an API gives me a headache.

Thanks, Richard

Can you try adding a user-agent header to the call…

I just tried that and it didn’t change anything, unfortunately.

Are you using a v2 client? Are you calling v2 endpoints?