diff --git a/include/boost/asio/detail/win_iocp_overlapped_ptr.hpp b/include/boost/asio/detail/win_iocp_overlapped_ptr.hpp
index 0d6c9a92..e8ab6b02 100644
--- a/include/boost/asio/detail/win_iocp_overlapped_ptr.hpp
+++ b/include/boost/asio/detail/win_iocp_overlapped_ptr.hpp
@@ -104,7 +104,8 @@ public:
     if (ptr_)
     {
       ptr_->ec_ = ec;
-      ptr_->io_service_.post_completion(ptr_, 0, bytes_transferred);
+      ptr_->io_service_.post_completion(ptr_, 0,
+          static_cast<DWORD>(bytes_transferred));
       ptr_ = 0;
     }
   }
diff --git a/include/boost/asio/detail/win_iocp_serial_port_service.hpp b/include/boost/asio/detail/win_iocp_serial_port_service.hpp
index 29c5a201..57d56cf6 100644
--- a/include/boost/asio/detail/win_iocp_serial_port_service.hpp
+++ b/include/boost/asio/detail/win_iocp_serial_port_service.hpp
@@ -237,7 +237,7 @@ public:
   }
 
   // Send a break sequence to the serial port.
-  boost::system::error_code send_break(implementation_type& impl,
+  boost::system::error_code send_break(implementation_type&,
       boost::system::error_code& ec)
   {
     ec = boost::asio::error::operation_not_supported;