diff --git a/lib/synth-audio.js b/lib/synth-audio.js
index 4b211c0..60aae1a 100644
--- a/lib/synth-audio.js
+++ b/lib/synth-audio.js
@@ -333,6 +333,7 @@ const synthPolly = async(createHash, retrieveHash, logger,
       }
     }
     if (region) params += `,region=${region}`;
+    if (engine) params += `,engine=${engine}`;
     params += '}';
 
     return {