diff --git a/src/functions/join.ts b/src/functions/join.ts
index 18e90a5..32dbc83 100644
--- a/src/functions/join.ts
+++ b/src/functions/join.ts
@@ -4,7 +4,7 @@ import { WithEncoding } from "./baseDefinitions";
 
 export function join(
     separator: string,
-    options?: WithEncoding & TransformOptions,
+    options: WithEncoding & TransformOptions = { encoding: "utf8" },
 ): Transform {
     let isFirstChunk = true;
     const decoder = new StringDecoder(options.encoding);