diff --git a/_includes/content/compose-var-sub.md b/_includes/content/compose-var-sub.md
index 4c333daab554..6b50d44c1098 100644
--- a/_includes/content/compose-var-sub.md
+++ b/_includes/content/compose-var-sub.md
@@ -34,7 +34,8 @@ provide inline default values using typical shell syntax:
 - `${VARIABLE-default}` evaluates to `default` only if `VARIABLE` is unset
   in the environment.
 
-Similarly, the following syntax allows you to specify mandatory variables:
+Similarly, the following syntax allows you to specify mandatory variables when
+using **Compose 1.19.0+** or `docker stack deploy` **18.04.0+**:
 
 - `${VARIABLE:?err}` exits with an error message containing `err` if
   `VARIABLE` is unset or empty in the environment.