diff --git a/lib/messages/moddn_request.js b/lib/messages/moddn_request.js
index 580e7a0..0f54823 100644
--- a/lib/messages/moddn_request.js
+++ b/lib/messages/moddn_request.js
@@ -60,7 +60,7 @@ ModifyDNRequest.prototype._toBer = function (ber) {
     const len = Buffer.byteLength(s)
 
     ber.writeByte(0x80) // MODIFY_DN_REQUEST_NEW_SUPERIOR_TAG
-    ber.writeByte(len)
+    ber.writeLength(len)
     ber._ensure(len)
     ber._buf.write(s, ber._offset)
     ber._offset += len