diff --git a/src/unix/lwt_unix.h b/src/unix/lwt_unix.h
index 125ca7a12..ab4ad64bf 100644
--- a/src/unix/lwt_unix.h
+++ b/src/unix/lwt_unix.h
@@ -14,6 +14,9 @@
 #include <caml/socketaddr.h>
 #include <string.h>
 
+// The following macro is for backwards compatibility.
+// It is given an `lwt_` prefix to avoid name collisions for code which
+// include both this file and alloc.h.
 #if OCAML_VERSION < 50000
 #define lwt_convert_flag_list(flags, table) \
     caml_convert_flag_list((flags), (int *)(table))