diff --git a/usr.sbin/vnconfig/vnconfig.8 b/usr.sbin/vnconfig/vnconfig.8
index 89b1b2f7744d..47e0e3db1af9 100644
--- a/usr.sbin/vnconfig/vnconfig.8
+++ b/usr.sbin/vnconfig/vnconfig.8
@@ -140,7 +140,10 @@ The list of allowed flags and their meanings are the same as for the
 option.
 .It Fl S Ar value{k,m,g,t}
 If no regular file is specified, VN will use swap for backing store.
-This option specifies the size of the device.  For example, '23m' for
+This option specifies the size of the device and implies the
+.Fl c
+option.
+For example, '23m' for
 23 megabytes.  The VN device will round the size up to a machine page boundary.
 Filesystems up to 7.9 terabytes are supported.  When specified along with
 a regular file, this option overrides the regular file's size insofar as