diff --git a/.gitignore b/.gitignore
index 81ce8be..bffb397 100644
--- a/.gitignore
+++ b/.gitignore
@@ -39,3 +39,5 @@ node_modules
 examples/*
 
 .vscode
+
+.env
\ No newline at end of file
diff --git a/lib/utils.js b/lib/utils.js
index 89b90c2..209d6c1 100644
--- a/lib/utils.js
+++ b/lib/utils.js
@@ -22,8 +22,8 @@ function makeSynthKey({account_sid = '', vendor, language, voice, engine = '', t
   hash.update(`${language}:${vendor}:${voice}:${engine}:${text}`);
   const hexHashKey = hash.digest('hex');
   const accountKey = account_sid ? `:${account_sid}` : '';
-  const extension = getFileExtension(vendor);
-  const key = `tts${accountKey}:${extension}:${hexHashKey}`;
+  const namespace = vendor.startsWith('custom') ? vendor : getFileExtension(vendor);
+  const key = `tts${accountKey}:${namespace}:${hexHashKey}`;
   return key;
 }