diff --git a/lib/synth-audio.js b/lib/synth-audio.js
index f44723d..b07994f 100644
--- a/lib/synth-audio.js
+++ b/lib/synth-audio.js
@@ -311,7 +311,7 @@ async function synthMicrosoft(logger, {
   text,
   filePath
 }) {
-  const synthFunc = process.env.ONPREM_AZURE_SYNTHESIZE_BY_HTTP ?
+  const synthFunc = process.env.JAMBONZ_ONPREM_AZURE_SYNTHESIZE_BY_HTTP ?
     _synthMicrosoftByHttp : _synthMicrosoftBySdk;
   return await synthFunc(logger, {
     credentials,