diff --git a/include/libtorrent/allocator.hpp b/include/libtorrent/allocator.hpp
index af9772547..6ee12d493 100644
--- a/include/libtorrent/allocator.hpp
+++ b/include/libtorrent/allocator.hpp
@@ -32,6 +32,9 @@ POSSIBILITY OF SUCH DAMAGE.
 
 #include <cstddef>
 
+#ifndef TORRENT_ALLOCATOR_HPP_INCLUDED
+#define TORRENT_ALLOCATOR_HPP_INCLUDED
+
 namespace libtorrent
 {
 
@@ -46,3 +49,5 @@ namespace libtorrent
 
 }
 
+#endif
+