diff --git a/man2/fsync.2 b/man2/fsync.2
index c20e34e42..8416eae16 100644
--- a/man2/fsync.2
+++ b/man2/fsync.2
@@ -139,6 +139,12 @@ to a value greater than 0.
 .\" -1: unavailable, 0: ask using sysconf().
 .\" glibc defines them to 1.
 .SH NOTES
+On some UNIX systems (but not Linux),
+.I fd
+must be a
+.I writable
+file descriptor.
+
 In Linux 2.2 and earlier,
 .BR fdatasync ()
 is equivalent to