When using a self-signed certificate, guzzle uses a multipart stream, which...

When using a self-signed certificate, guzzle uses a multipart stream, which doesn't allow arrays, so they need to be encoded to JSON first.
[skip ci]

src/Request.php

@@ -183,6 +183,7 @@ class Request
         //Reformat data array in multipart way if it contains a resource
         foreach ($data as $key => $item) {
             $has_resource |= is_resource($item);
+            is_array($item) && $item = json_encode($item);
             $multipart[] = ['name' => $key, 'contents' => $item];
         }
         if ($has_resource) {