diff --git a/doc/control-spec.txt b/doc/control-spec.txt
index 0e793b9ac1..2cacf4060c 100644
--- a/doc/control-spec.txt
+++ b/doc/control-spec.txt
@@ -242,12 +242,12 @@ the message.
   with an internal ERROR message.
 
   The client may decline to provide a body for the original address, and
-  instead send a special null address (0.0.0.0 for IPv4, ::0 for IPv6, or "."
-  for hostname).  This signifies that the server should choose the original
-  address itself, and return that address in the ADDRESSMAPPED message.  The
-  server should an element of address space that is unlikely to be in actual
-  use.  If there is already an address mapped to the destination address, the
-  server may reuse that mapping.
+  instead send a special null address (0.0.0.0 for IPv4, ::0 for IPv6, or
+  "."  for hostname).  This signifies that the server should choose the
+  original address itself, and return that address in the ADDRESSMAPPED
+  message.  The server should ensure that an element of address space
+  that is unlikely to be in actual use.  If there is already an address
+  mapped to the destination address, the server may reuse that mapping.
 
   If the original address is already mapped to a different address, the old
   mapping is removed.  If the original address and the destination address