diff --git a/processing/speechtotext/speechtotext.js b/processing/speechtotext/speechtotext.js
index 8f7b490f..0b110ed7 100644
--- a/processing/speechtotext/speechtotext.js
+++ b/processing/speechtotext/speechtotext.js
@@ -66,17 +66,18 @@ function onChallengeRequest(challenge, secret, signature) {
     console.log('[OK] Challenge accepted!');
     return {
       headers: {
-        // force Content-Type as JSON as Watson sends an Accept: application/json
-        'Content-Type': 'application/json'
+        'Content-Type': 'text/plain'
       },
-      // but sends the body as raw data as Watson expects plain text
-      body: new Buffer(challenge).toString('base64')
+      body: challenge
     };
   }
 
   console.log('[KO] Signature does not match');
   return {
     code: 500,
+    headers: {
+      'Content-Type': 'text/plain'
+    },
     body: 'Bad signature'
   };
 }