diff --git a/ii-doc.txt b/ii-doc.txt
index e6f4b36..92ac315 100644
--- a/ii-doc.txt
+++ b/ii-doc.txt
@@ -124,7 +124,7 @@ Implementation notes
 * Most HTTP servers are configured to reject long GET lines, so tii passes a
   limited amount of message IDs to the /u/m endpoints. This behaviour can be
   configured in the stations.txt file.
-* Some of the crucial validations is for message IDs to be 20 lines and for
+* Some of the crucial validations are for message IDs to be 20 lines and for
   all messages (Node-to-Point and Point-to-Node) to strictly have LF line
   endings, not CRLF.
 * The message order in an echo does not always match the timestamp ordering;