diff --git a/lib/tts-client/stepfun.ts b/lib/tts-client/stepfun.ts
index 37ce8c0..1597fcf 100644
--- a/lib/tts-client/stepfun.ts
+++ b/lib/tts-client/stepfun.ts
@@ -13,7 +13,7 @@ function arrayBufferToBase64(buffer: ArrayBuffer): string {
   let binary = "";
   const len = bytes.byteLength;
   for (let i = 0; i < len; i++) {
-    binary += String.fromCharCode(bytes[i]);
+    binary += String.fromCharCode(bytes[i]!);
   }
   return btoa(binary);
 }