diff --git a/doc/hamnet70.adoc b/doc/hamnet70.adoc
index 35cb44c..b356639 100644
--- a/doc/hamnet70.adoc
+++ b/doc/hamnet70.adoc
@@ -245,7 +245,7 @@ It is used in two cases:
   Here, _TX Request_ is set to 0.
 
 Empty Frames are not acknowledged.
-They therefore do not have a _TX sequence number_; this field is reserved and should be set to zero.
+They therefore do not have a _TX sequence number_; this field is reserved and must be set to zero.
 
 The _RX sequence number_ and all other header fields are used as usual.