diff --git a/DOCS/tech/nut.txt b/DOCS/tech/nut.txt
index 810277e250..b1e2ea6911 100644
--- a/DOCS/tech/nut.txt
+++ b/DOCS/tech/nut.txt
@@ -505,6 +505,7 @@ time_base_nom / time_base_denom = time_base
 
 time_base_id
     index into the time_base table
+    MUST by < time_base_count
 
 convert_ts
     To switch from 2 different timebases, the following calculation is