diff --git a/lib/oauthlib.php b/lib/oauthlib.php
index b84c882e795..318f13a602d 100644
--- a/lib/oauthlib.php
+++ b/lib/oauthlib.php
@@ -510,6 +510,10 @@ abstract class oauth2_client extends curl {
 
         $r = json_decode($response);
 
+        if (is_null($r)) {
+            throw new moodle_exception("Could not decode JSON token response");
+        }
+
         if (!isset($r->access_token)) {
             return false;
         }
@@ -552,6 +556,9 @@ abstract class oauth2_client extends curl {
             }
         }
 
+        // Force JSON format content in response.
+        $this->setHeader('Accept: application/json');
+
         return parent::request($murl->out(false), $options);
     }