diff --git a/src/idz/util/bin.ts b/src/idz/util/bin.ts
index 79cf7c2..a28a5a8 100644
--- a/src/idz/util/bin.ts
+++ b/src/idz/util/bin.ts
@@ -33,7 +33,7 @@ function writeStr(
   str: string,
   encoding: string
 ) {
-  const tmp = iconv.encode(str + "\0", encoding);
+  const tmp = iconv.encode(str, encoding);
 
   if (tmp.length >= end - begin) {
     throw new Error("String field overflow");