diff --git a/cpp/platform/impl/windows/bluetooth_classic_socket.cc b/cpp/platform/impl/windows/bluetooth_classic_socket.cc
index c54a3d9a..786ad8b7 100644
--- a/cpp/platform/impl/windows/bluetooth_classic_socket.cc
+++ b/cpp/platform/impl/windows/bluetooth_classic_socket.cc
@@ -119,8 +119,8 @@ BluetoothSocket::BluetoothOutputStream::BluetoothOutputStream(
 
 Exception BluetoothSocket::BluetoothOutputStream::Write(const ByteArray& data) {
   Buffer buffer = Buffer(data.size());
-
   std::memcpy(buffer.data(), data.data(), data.size());
+  buffer.Length(data.size());
 
   try {
     winrt_stream_.WriteAsync(buffer).get();