diff --git a/include/serial/impl/unix.h b/include/serial/impl/unix.h
index 3e45ec4..88f8b02 100644
--- a/include/serial/impl/unix.h
+++ b/include/serial/impl/unix.h
@@ -44,6 +44,7 @@
 
 namespace serial {
 
+using std::size_t;
 using std::string;
 using std::invalid_argument;