diff --git a/share/man/man9/mbuf.9 b/share/man/man9/mbuf.9
index 9e325a28be37..4ba28675cc66 100644
--- a/share/man/man9/mbuf.9
+++ b/share/man/man9/mbuf.9
@@ -532,6 +532,10 @@ and other mbuf-reliant parts of the kernel.
 causes ip_output to fragment outgoing mbuf chains into fragments of
 the specified size.  Setting this option to 1 is an excellent way to
 test the long mbuf chain handling ability of network drivers.
+.Pp
+.Va kern.ipc.m_defragrandomfailures
+causes the function m_defrag to randomly fail, returning NULL.  Any
+piece of code which uses m_defrag should be tested with this feature.
 .El
 .Sh RETURN VALUES
 See above.